Method for manufacturing a ceramic part, in particular synthetic diamond

Abstract

The invention relates to a method for producing a ceramic part from a ceramic powder sample, the method comprising: • - the sample being heated to a predetermined minimum temperature higher than a sintering temperature of the ceramic powder. • - a plurality of acoustic wave generators generating a plurality of acoustic waves and transmitting these waves to a focusing system, each acoustic wave generator being configured to transmit an acoustic wave of frequency f of between 1 MHz and 100 MHz, the acoustic waves together producing an acoustic intensity I <sb / > e <sb / > at the input of the focusing system; • - the focusing system focusing the acoustic waves into a focal zone in the sample, the focusing system defining a multiplication factor m, the focused acoustic waves together producing, in at least one portion of the focal zone, a minimum acoustic intensity of at least M x le, at least equal to M x le = 1.1012 W.m-2, which, when combined with the minimum heating temperature applied to the ceramic powder sample, is sufficient to sinter the ceramic powder in the portion of the focal zone.

Description

[0001] Process for manufacturing a ceramic part, in particular synthetic diamond

[0002] Technical field of the invention

[0003] The present invention falls within the field of the production of a ceramic part, in particular diamond.

[0004] The invention relates in particular to a method for manufacturing a ceramic part by sintering, preferably a method for manufacturing synthetic diamond, as well as a device for implementing this method.

[0005] Prior art

[0006] Ceramic parts produced by sintering are increasingly used in industry, particularly for their electronic, mechanical, optical, thermal and chemical properties.

[0007] Conventional sintering is a process that involves heating a powder of a material chosen to be the basis of the ceramic (alumina, zirconia for example). Heating allows the grains of the powder to create chemical bonds between them and thus form a ceramic. The temperatures required are generally above 1500°C and the heating times are around several hours (10°C per minute at the core). The main disadvantage of conventional sintering is that it produces ceramic parts with grain sizes of around 10 to 100 μm which are hard but have low mechanical strength. Indeed, the grain size of a mechanical part is one of the parameters that determine the mechanical strength of said part. The smaller the grain size, the higher the mechanical strength of the mechanical part.

[0008] During the 20th èmecentury, conventional sintering was improved to allow faster heating of powders, up to 200°C per minute at the core. This is called flash sintering ("Spark Plasma Sintering" in the English literature). Flash sintering consists of placing the ceramic powder in a mold, generally made of graphite, and passing a high-intensity direct or alternating current through the mold and into the powder if the material is conductive. This innovation made it possible to reduce sintering times to around ten minutes and thus limit the growth of the powder grains and promote the creation of chemical bonds between these grains. This advantage makes it possible to produce mechanical parts with grain sizes of around 1 μm by heating to temperatures above 1200°C.

[0009] One of the disadvantages of flash sintering is that it requires relatively high heating temperatures. This therefore limits the use of materials that are metastable at room temperature and pressure for the production of mechanical parts. For example, diamond is stable at room temperature and pressure but reverts to graphite when heated above 800°C in an oxygenated atmosphere and above 1700°C in an inert atmosphere or vacuum.

[0010] At the beginning of the 21st èmecentury, flash sintering technology was improved by compressing powders at pressures exceeding several hundred MPa. Adding such pressure to flash sintering not only makes it possible to sinter metastable materials at ambient pressure but also to lower the temperature required for powder sintering, between 500 and 1000°C depending on the material, and thus further reduce grain growth during sintering. Mechanical parts with grain sizes below 100nm have been obtained in around ten minutes. It is also possible to sinter nanometric diamond powders at 5 GPa and 1500°C to obtain polycrystalline diamond mechanical parts with grains of 10 to 100nm.

[0011] One of the main disadvantages of high-pressure flash sintering is the impossibility of producing complex shapes during sintering. Indeed, the devices currently used to achieve pressures above GPa severely constrain the shape of the sintering mold.

[0012] To produce synthetic diamond ceramic parts, other technologies, other than high-pressure flash sintering of diamond powder, exist.

[0013] Among these technologies, we can mention High Pressure, High Temperature technology, known by the acronym HPHT ("High Pressure High Temperature" in English literature). This technology consists of placing a carbon sample, for example graphite, in an enclosure and subjecting it to a high temperature, of the order of 1500°C, and a high pressure, of the order of 15 GPa. The addition of a catalyst to the sample, for example a transition metal such as nickel or cobalt, allows the temperature to be lowered to 1000°C and the pressure to 6 GPa. HPHT technology allows, similar to the pressure flash sintering technology, to sinter diamond pellets, but starting from graphite powder instead of diamond powder. In this case, the graphite powder is compressed to more than 7 GPa and heated to more than 2000°C.The powder then successively undergoes the stages of change of state and sintering to form a diamond pellet.

[0014] Another technology involves chemical vapor deposition, known as CVD (Chemical Vapor Deposition). A diamond sample is placed in a vacuum chamber. A gas, including hydrogen, is introduced into the chamber. The whole is then ionized using a microwave discharge, creating a plasma. The species from the plasma gradually adsorb onto the substrate and form the object.

[0015] For these first two technologies, the main disadvantages are:

[0016] - the difficulty, or even impossibility, of finely controlling the spatial location where the conditions are met to allow the formation of diamonds;

[0017] - the limitation of the diamond growth direction: for HPHT technology, the directions are imposed by the compression pattern; for CVD technology, the direction is perpendicular to the substrate.

[0018] Another technology, which allows the production of only synthetic diamond powder, is based on the principle of dynamic compression to convert graphite into diamond.

[0019] In dynamic compression-induced graphite-to-diamond conversion technologies, the conversion is not diffusive, as in HPHT technology, i.e. it is not induced by the diffusion of atoms under the effect of temperature. The conversion is called martensitic, that is to say it is induced by the local deformation of the crystal lattices, as described in particular in the publication of Christian et al, December 1995, Journal de Physique IV, volume 5, number C8, and as demonstrated by numerous experiments since the beginning of the 1990s, in particular in the publication of Erskine et al, 1992, Journal of Applied Physics, vol 71, n°10, or in the publication of Kraus et al., 2016, Nature Communications, 7, 10970 or that of Volt et al., 2019, Journal of Applied Physics, 125, 245902.

[0020] This technology relies, for example, on the use of shock waves. Shock waves are generated, for example, by projecting a metal foil onto a graphite sample. In the literature, we generally refer to shock waves by explosion. Shock waves can also be generated by laser ablation. This technology has many disadvantages, including the partial destruction of the graphite sample to produce these shocks and the very limited duration of these shocks, on the order of microseconds. The diamonds obtained are only in the form of nanocrystals or microcrystals.

[0021] Presentation of the invention

[0022] The present invention aims to remedy the drawbacks of the solutions proposed by the prior art, in particular those set out above.

[0023] To this end, the present invention provides a method for producing a ceramic part from a sample of ceramic powder, comprising:

[0024] - heating, by a heating system, the ceramic powder sample to a predetermined minimum temperature, then

[0025] - the generation and emission, by a plurality of acoustic wave generators, of a plurality of acoustic waves, towards a focusing system, each acoustic wave generator being configured to emit an acoustic wave of frequency f between 1 MHz and 100 MHz, said acoustic waves together generating at the input of the focusing system an acoustic intensity I e ,

[0026] - focusing said acoustic waves, by the focusing system, into a focal zone in the ceramic powder sample, the focusing system defining a multiplicative factor M.

[0027] Said focused acoustic waves together generate, at at least a portion of the focal zone, a minimum acoustic intensity of at least M x le, sufficient to, combined with the minimum heating temperature applied to the ceramic powder sample, sinter the ceramic powder in said portion of the focal zone.

[0028] Ceramics are understood to mean so-called technical ceramics. Technical ceramics include:

[0029] - ceramic oxides, which consist mainly of metal oxide such as, for example, zirconium oxide or alumina; - non-oxide ceramics which are materials based on carbon, nitride and silicone compounds such as silicon carbide, tungsten carbide or aluminium nitride;

[0030] - silicate-based ceramics, mainly made from raw materials combined with aluminas such as aluminum silicate.

[0031] The method according to the invention advantageously makes it possible to heat a ceramic powder to a predefined minimum temperature, while compressing it locally to a pressure such that the sintering of the ceramic powder can take place.

[0032] It is clear from the method that the heating of the sample is carried out prior to the generation and emission of the acoustic waves by the plurality of acoustic wave generators. The heating of the sample does not come at all from the acoustic waves emitted by the plurality of acoustic wave generators.

[0033] The predefined minimum temperature is preferably higher than the sintering temperature of the selected ceramic powder. The sintering temperature corresponds to the minimum temperature necessary for the sintering of the powder at a given pressure. The higher the pressure applied, the lower the sintering temperature.

[0034] It is known that sound pressure can be expressed as a function of sound intensity. For a given material initially at room temperature and pressure, each sound intensity value gives a pressure. Thus, knowing the pressure as a function of sound intensity and the minimum temperature required for sintering as a function of pressure for the same powder, it is possible to access the minimum temperature required for sintering as a function of sound intensity.

[0035] Thus, depending on the ceramic powder used in the process according to the invention, it is possible to associate with it a pair of values ​​(acoustic intensity, heating temperature) to be applied which advantageously allows the sintering of the powder in at least one part of the focal zone.

[0036] In other words, the sintering temperature of a ceramic powder is a function of the acoustic intensity that is generated at the at least part of the focal zone. The acoustic intensity and the sintering temperature required to sinter a ceramic powder depend on the ceramic powder used. From the prior knowledge of the acoustic intensity that can be generated, at the at least part of the focal zone, by the focused acoustic waves, the sintering temperature can be deduced. The method according to the invention can then be applied, for a given ceramic powder, with the pair of values ​​(acoustic intensity, heating temperature) to be applied, in order to be able to sinter the ceramic powder in the at least part of the focal zone.

[0037] The method according to the invention thus advantageously makes it possible to create sintering of the ceramic powder at the level of a part of the focal zone. The method advantageously makes it possible to finely control the spatial location where the conditions are met to allow the sintering of this ceramic powder.

[0038] In an exemplary implementation, the method according to the invention allows, from an acoustic intensity of 5.10 11 W.nr 2 , and a minimum heating temperature of 1360°C, to obtain diamond by sintering diamond powder.

[0039] According to particular embodiments, the invention further meets the following characteristics, implemented separately or in each of their technically effective combinations.

[0040] In particular embodiments of the invention, the ceramic powder is chosen from: a diamond powder, a tungsten carbide powder, a silicon nitride powder, a fully stabilized zirconia powder, an alpha alumina powder, a gamma alumina powder, and a titanium oxide powder.

[0041] In particular embodiments of the invention, the method comprises, during the emission of the acoustic waves, the displacement of the focal zone in the sample.

[0042] Such a displacement of the focal zone in the sample advantageously allows the sample to be sintered in selected areas to produce a part with a determined shape from the powder.

[0043] The invention also relates to a device for implementing the method in accordance with at least one of its modes of implementation, said device comprising:

[0044] - a heating system intended to heat the sample,

[0045] - a plurality of acoustic wave generators, each configured to generate an acoustic wave, the plurality of acoustic wave generators and the sample heating system being separate elements in said device,

[0046] - a system for focusing acoustic waves into a focal zone in the sample, the focusing system defining the multiplicative factor M.

[0047] Each acoustic wave generator of the device is advantageously configured to emit an acoustic wave, of frequency f, preferably between 1 MHz and 100 MHz.

[0048] In a preferred embodiment, all of the acoustic wave generators of the device emit at the same frequency.

[0049] The focusing system is arranged and sized to receive all the acoustic waves emitted by the acoustic wave generators.

[0050] According to particular embodiments, the invention further meets the following characteristics, implemented separately or in each of their technically operative combinations.

[0051] In particular embodiments, the heating system allows heating of the sample by Joule effect or by inductive effect.

[0052] In particular embodiments, each acoustic wave generator is a piezoelectric actuator.

[0053] In particular embodiments, to sinter the entire sample, the device comprises a member for moving the focal zone in the sample.

[0054] In embodiments of the displacement member, said member for displacing the focal zone in the sample is a member for displacing the acoustic wave focusing system and / or a member for displacing the sample and / or a member for displacing the acoustic wave generators and / or a member for phase shifting the acoustic wave generators.

[0055] The invention also relates to a method for producing diamond from a sample comprising a graphite powder, comprising:

[0056] - heating, by a heating system, the graphite powder sample to a predetermined minimum temperature, then

[0057] - the generation and emission, by a plurality of acoustic wave generators, of a plurality of acoustic waves, towards a focusing system, each acoustic wave generator being configured to emit an acoustic wave of frequency f between 1 MHz and 100 MHz, said acoustic waves together generating at the input of the focusing system an acoustic intensity I e ,

[0058] - focusing said acoustic waves, by the focusing system, into a focal zone in the graphite powder sample, the focusing system defining a multiplicative factor M, said focused acoustic waves generating together, at at least part of the focal zone, an acoustic intensity at least equal to M x le = 1.10 12 W.rrr 2corresponding to a minimum acoustic intensity sufficient to: o in a first phase, carry out a phase change of the graphite powder into diamond powder, then o in a second phase, combined with the minimum heating temperature applied to the sample (50), sinter the diamond powder obtained.

[0059] To achieve a phase change from graphite to diamond, at the focal zone, the acoustic intensity must be sufficient so that:

[0060] - the acoustic pressure reached exceeds a sufficient minimum pressure making the diamond more stable than graphite,

[0061] - the kinetic energy of the atoms is sufficient to break the metastability of said graphite during the duration of the emission of the acoustic waves.

[0062] When the acoustic intensity generated at the focal zone is at least equal to 1.10 12 W.rrr 2, the duration of the acoustic intensity emission then conditions the percentage of conversion of graphite into diamond in this focal zone. In other words, for a given acoustic intensity emitted at the focal zone, the greater the duration of the acoustic intensity emission, the greater the percentage of conversion of graphite into diamond in the focal zone. In addition, for a desired percentage of conversion of graphite into diamond, the greater the acoustic intensity emitted, the smaller the duration of the acoustic intensity emission at the focal zone.

[0063] Thus, depending on the acoustic intensity generated at the focal zone, the acoustic waves are preferentially emitted for at least a minimum duration allowing a desired predefined percentage of conversion of graphite into diamond in the focal zone. It is clear from the method that the heating of the sample is carried out prior to the generation and emission of the acoustic waves by the plurality of acoustic wave generators. The heating of the sample does not come from the acoustic waves emitted by the plurality of acoustic wave generators. By heating the sample in parallel, to a predefined minimum temperature, in the part of the focal zone where the acoustic intensity reaches 1.10 12 W.nr 2 , the diamond powder obtained is advantageously densified by sintering.

[0064] The sintering temperature of graphite powder is a function of the acoustic intensity which is generated at least in part of the focal zone.

[0065] From the prior knowledge of the acoustic intensity that can be generated, at the level of at least one part of the focal zone, by the focused acoustic waves, the sintering temperature can be deduced. The method according to the invention can then be applied, for the graphite powder, with the pair of values ​​(acoustic intensity, heating temperature) to be applied, in order to be able to carry out the sintering of the graphite powder in the at least one part of the focal zone.

[0066] The invention also relates to a method for producing diamond from a sample comprising an amorphous carbon powder, comprising:

[0067] - heating the amorphous carbon powder sample to a predetermined minimum temperature, then

[0068] - the generation and emission, by a plurality of acoustic wave generators, of a plurality of acoustic waves, towards a focusing system, each acoustic wave generator being configured to emit an acoustic wave of frequency f between 1 MHz and 100 MHz, said acoustic waves together generating at the input of the focusing system an acoustic intensity I e ,

[0069] - focusing said acoustic waves, by the focusing system, into a focal zone in the sample of amorphous carbon powder, the focusing system defining a multiplicative factor M, said focused acoustic waves generating together, at least in part of the focal zone, an acoustic intensity at least equal to M x le = 2.1, 10 12 W.rrr 2corresponding to a minimum acoustic intensity sufficient to: o in a first phase, carry out a phase change of the amorphous carbon powder into diamond powder, then o in a second phase, combined with the minimum heating temperature applied to the sample, sinter the diamond powder obtained.

[0070] By "amorphous carbon" is meant an allotropic form of carbon comprising a mixture of sp-type atomic bonds 2 (same type as graphite) and sp-type atomic bonds 3 (same type as diamond), as well as a possible concentration of hydrogen atoms. The distribution of the different types of bond is random. This distribution does not necessarily present any order on a small or large scale.

[0071] Advantageously, amorphous carbon, without hydrogen atoms, generally has a sp type hybridization rate 3 between 40% and 90%.

[0072] To achieve a phase change from amorphous carbon to diamond, at the focal zone, the acoustic intensity must be sufficient so that:

[0073] - the acoustic pressure reached exceeds a sufficient minimum pressure making the diamond more stable than graphite,

[0074] - the kinetic energy of the atoms is sufficient to break the metastability of said graphite during the duration of the emission of the acoustic waves.

[0075] When the acoustic intensity generated at the focal zone is at least equal to 2.1 .10 12 W.rrr 2, the duration of the acoustic intensity emission then conditions the percentage of conversion of amorphous carbon into diamond in this focal zone. In other words, for a given acoustic intensity emitted at the focal zone, the greater the duration of the acoustic intensity emission, the greater the percentage of conversion of amorphous carbon into diamond in the focal zone. Moreover, for a desired percentage of conversion of amorphous carbon into diamond, the greater the acoustic intensity emitted, the smaller the duration of the acoustic intensity emission at the focal zone.

[0076] Thus, depending on the acoustic intensity generated at the focal zone, the acoustic waves are preferentially emitted for at least a minimum duration allowing a desired predefined percentage of conversion of the amorphous carbon into diamond in the focal zone. It is clear from the method that the heating of the sample is carried out prior to the generation and emission of the acoustic waves by the plurality of acoustic wave generators. The heating of the sample does not come from the acoustic waves emitted by the plurality of acoustic wave generators.

[0077] By heating the sample in parallel, to a predefined minimum temperature, in the part of the focal zone where the acoustic intensity reaches 2, 1.10 12 W.nr 2 , the diamond powder obtained is densified by sintering.

[0078] The sintering temperature of amorphous carbon powder is a function of the acoustic intensity that is generated at least in part of the focal zone.

[0079] From the prior knowledge of the acoustic intensity that can be generated, at the level of at least one part of the focal zone, by the focused acoustic waves, the sintering temperature can be deduced. The method according to the invention can then be applied, for the amorphous carbon powder, with the pair of values ​​(acoustic intensity, heating temperature) to be applied, in order to be able to carry out the sintering in the at least one part of the focal zone.

[0080] The invention also relates to a device for implementing the method for producing diamond from a sample comprising a graphite powder or from a sample comprising an amorphous carbon powder, said device comprising:

[0081] - a heating system intended to heat the sample,

[0082] - a plurality of acoustic wave generators, each configured to generate an acoustic wave, the plurality of acoustic wave generators and the sample heating system being separate elements in said device,

[0083] - a system for focusing acoustic waves into a focal zone in the sample, the focusing system defining the multiplicative factor M.

[0084] Each acoustic wave generator of the device is advantageously configured to emit an acoustic wave, of frequency f, preferably between 1 MHz and 100 MHz.

[0085] In a preferred embodiment, all the acoustic wave generators of the device emit at the same frequency. The focusing system is arranged and sized to receive all the acoustic waves emitted by the acoustic wave generators.

[0086] According to particular embodiments, the invention further meets the following characteristics, implemented separately or in each of their technically operative combinations.

[0087] In particular embodiments, the heating system allows heating of the sample by Joule effect or by inductive effect.

[0088] In particular embodiments, each acoustic wave generator is a piezoelectric actuator.

[0089] In particular embodiments, to make the diamond appear, the device comprises a member for moving the focal zone in the sample.

[0090] In embodiments of the displacement member, said member for displacing the focal zone in the sample is a member for displacing the acoustic wave focusing system and / or a member for displacing the sample and / or a member for displacing the acoustic wave generators and / or a member for phase shifting the acoustic wave generators.

[0091] Brief description of the figures

[0092] The invention will be better understood on reading the following description, given by way of non-limiting example, and made with reference to the following figures: Figure 1 represents a perspective view of a device adapted in particular to the method of manufacturing a ceramic according to the invention;

[0093] Figure 2 represents a perspective view of one half of the device of Figure 1; Figure 3 represents a cross-section of the device of Figure 2;

[0094] Figure 4 represents an enlargement of zone III of Figure 3, illustrating an exemplary embodiment of a piezoelectric actuator;

[0095] Figure 5 shows a graph illustrating the sintering temperature as a function of pressure, for different types of ceramic powder;

[0096] Figure 6 shows a graph illustrating the sintering temperature as a function of the acoustic intensity, for different types of ceramic powder,

[0097] Figure 7 represents a modeling curve illustrating the induced pressure as a function of the acoustic intensity, for graphite and amorphous carbon;

[0098] Figure 8 represents a modeling curve illustrating the kinetic energy as a function of the acoustic intensity, for graphite and amorphous carbon; Figure 9 represents a graph illustrating the duration necessary for the emission of an acoustic wave as a function of the intensity of said acoustic wave to recreate the conditions sufficient to achieve a 99% phase change of a graphite sample into diamond and of an amorphous carbon sample into diamond.

[0099] In these figures, like reference numerals from one figure to another designate identical or similar elements. Furthermore, for reasons of clarity, the drawings are not to scale unless otherwise indicated.

[0100] Description of the embodiments

[0101] A device 100 is illustrated in Figures 1 to 4. This device 100 is suitable for performing:

[0102] - a method for manufacturing a ceramic part from a sample 50 of ceramic powder; this method will hereinafter be referred to as the first method;

[0103] - a method of manufacturing a diamond from a sample 50 comprising a graphite or amorphous carbon powder; this method will hereinafter be referred to as the second method.

[0104] In the case of the first method, sample 50 contains only ceramic powder.

[0105] The ceramic powder is preferably chosen from a powder of tungsten carbide, silicon nitride, fully stabilized zirconia (known by the acronym FSZ for “fully stabilized zirconia”), alpha alumina, gamma alumina, titanium oxide, diamond.

[0106] In the case of the second method, the sample 50 may comprise a diamond seed and graphite or amorphous carbon powder. Preferably, the sample 50 comprises only graphite or amorphous carbon powder. Preferably, the ceramic powder or the graphite or amorphous carbon powder consists of nanometric-sized grains. The grains are dissociated from each other.

[0107] In the remainder of the description, the term "grains" means particles whose ratio of the largest dimension to the smallest dimension is preferably less than or equal to 5. The grains may be spherical, cubic or other geometric shapes. The term "size" as applied to grains means the largest dimension of these grains.

[0108] The term "nanometric" also means a dimension greater than or equal to 1 nanometer and less than 1000 nanometers.

[0109] The device 100 comprises, as illustrated in Figures 1 to 4:

[0110] - a plurality of acoustic wave generators 10,

[0111] - a focusing system 20 of said acoustic waves into a focal zone in the sample 50.

[0112] Preferably, as illustrated in Figures 3 and 4, the acoustic wave generators 10 are attached to the focusing system 20.

[0113] The sample 50, located downstream of the focusing system 20, is placed in a medium, called the propagation medium 40. The sample 50 is held in position in the propagation medium 40. A container 70, placed in the propagation medium 40, makes it possible, for example, to contain the sample 50.

[0114] In one embodiment, the container 70 is in the form of a tube, closed at at least one of its two ends, preferably closed at both of its ends.

[0115] Each acoustic wave generator 10 of the device 100 is configured to generate and emit an acoustic wave, of frequency f, preferably between 1 MHz and 100 MHz.

[0116] In a preferred embodiment, all the acoustic wave generators 10 of the device 100 emit at the same frequency.

[0117] Preferably, the acoustic waves emitted by the generators 10 are synchronous.

[0118] In another non-limiting example of embodiment, a portion of the acoustic wave generators 10 of the device 100, for example half, emit at the same frequency fi and the other acoustic wave generators 10 of the device 100 emit at the same frequency f2, different from fi.

[0119] Each acoustic wave generator 10 of the device 100 is configured to generate and emit an acoustic wave, of power P.

[0120] Preferably, the acoustic wave generators 10 are similar to each other. In a preferred embodiment, the acoustic wave generators 10 are piezoelectric actuators. A piezoelectric actuator converts an electrical signal into an acoustic signal, at a given power. In a preferred embodiment of a piezoelectric actuator, said piezoelectric actuator comprises a successive stack of layers:

[0121] - at least one impedance mismatch layer 11,

[0122] - at least one piezoelectric element 12,

[0123] - at least one impedance matching layer 13.

[0124] The at least one piezoelectric element 12 is thus interposed between the at least one impedance mismatching layer 11 and the at least one impedance matching layer 13. The at least one impedance matching layer 13 is, for its part, interposed between the at least one piezoelectric element 12 and the focusing system 20.

[0125] In the non-limiting example illustrated in FIG. 4, the piezoelectric actuator 10 comprises two impedance mismatching layers 11, a piezoelectric element 12 and an impedance matching layer 13.

[0126] The frequency of the electrical signal applied to the terminals of the at least one piezoelectric element 12 determines the frequency of the generated acoustic wave. This frequency may, for example, correspond to the resonance frequency of the at least one piezoelectric element 12 to maximize the conversion rate of the electrical signal into an acoustic wave.

[0127] The at least one impedance matching layer 13 is preferentially configured to maximize the transmission of the acoustic wave from the at least one piezoelectric element 12 to the focusing system 20.

[0128] The at least one impedance mismatch layer 11 is preferentially configured to minimize transmission of the acoustic wave out of the device 100.

[0129] The piezoelectric actuators 10 are preferably arranged and held fixedly in a part, called barrel 15.

[0130] Said barrel comprises a plurality of orifices 151 each intended to receive a piezoelectric actuator. Said orifices pass through the thickness of the barrel.

[0131] Each piezoelectric actuator 10 is arranged in an orifice 151 of the barrel 15 so that the at least one impedance matching layer 13 faces the focusing system 20.

[0132] Said orifices of the barrel 15 are preferably parallel to each other. The piezoelectric actuators 10 being arranged parallel to each other, the acoustic waves at the output of the piezoelectric actuators propagate in the same direction.

[0133] Said orifices are preferably arranged in the barrel 15 so that the piezoelectric actuators 10 are arranged equidistant from each other.

[0134] The constituent elements of the piezoelectric actuators 10 are for example of cylindrical shape. The orifices 151 of the barrel 15 are of cylindrical shape.

[0135] Preferably, a prestressing bolt 16 is configured to hold and compress a piezoelectric actuator 10 in an orifice 151 of the barrel 15. Such compression on the piezoelectric actuator advantageously makes it possible to maximize the conversion rate of electrical energy into acoustic energy. Each prestressing bolt 16 preferably bears on the impedance mismatch layer 11.

[0136] The device 100 comprises as many prestressing bolts 16 as piezoelectric actuators 10.

[0137] The device 100 preferably comprises a part, called a cover 17, fixedly attached to the barrel 15. Said cover comprises threaded orifices 171 into which the prestressing bolts 16 are screwed.

[0138] The device 100 preferably comprises a management unit (not shown in the figures) for the acoustic wave generators 10.

[0139] The device 100, preferably the management unit, is configured to control the duration of the emission of the acoustic waves by the plurality of acoustic wave generators 10.

[0140] In an exemplary embodiment (not shown in the figures), the management unit comprises a power source and a synchronization unit.

[0141] Said power source is configured to electrically power each of the acoustic wave generators 10, in the form of a continuous signal.

[0142] Said synchronization unit is configured to drive the acoustic wave generators 10 synchronously.

[0143] In a non-limiting example of embodiment, when the acoustic wave generators 10 are piezoelectric actuators, the management unit comprises:

[0144] - a power source configured to provide the power required by each of the piezoelectric actuators, in the form of a continuous signal;

[0145] - a synchronization unit comprising: o a first clock, called primary, to control the piezoelectric actuators from a single time reference; o cascaded clocks to control the power circuits of each of the piezoelectric actuators with in-phase signals; o power circuits consisting of half-H bridges to charge and discharge the piezoelectric actuators.

[0146] As described previously, the device 100 further comprises a focusing system 20.

[0147] Such a focusing system 20 is configured to receive and focus the acoustic waves, emitted by the plurality of acoustic wave generators 10, into a focal zone in the sample 50.

[0148] By focal zone we mean a region of reduced dimension, where all the acoustic waves will converge.

[0149] The focal zone is preferably smaller than that of sample 50.

[0150] Preferably, the sample is contained in a container 70 in the form of a tube with a diameter of 50 mm and a length of 50 mm. The sample completely fills the container.

[0151] Preferably, the focal area is less than 1 mm x 1 mm x 1 mm in size.

[0152] The dimensions of the focal zone are fixed by the frequency of the acoustic waves, the propagation speed of the acoustic waves in the sample 50, the diameter of the cross-section of the focusing system 20 and the focal length of the focusing system 20. By cross-section is meant the section which is perpendicular to an axis of symmetry of the focusing system.

[0153] The focal zone is generally comparable to an ellipsoid, with a circular cross-section. The cross-section of the focal zone is understood to mean the section that is perpendicular to the direction of propagation of an acoustic wave located in an axis of symmetry of the focusing system. The ellipsoid is defined by its semi-major axis and its semi-minor axis. The semi-major axis corresponds to half the length of the ellipsoid, in the direction of propagation of an acoustic wave located in an axis of symmetry of the focusing system. The semi-minor axis corresponds to the radius of the maximum cross-section.Preferably, the focal zone is an ellipsoid, of circular cross-section, with a semi-major axis between 0.2 mm and 2 mm and a semi-minor axis between 0.05 mm and 1 mm, even more preferably between 0.07 mm and 0.75 mm, the high values ​​applying to powders having high acoustic speeds, such as for example diamond powder, and the low values ​​applying to powders having low acoustic speeds, such as for example graphite powder.

[0154] Preferably, the sample is initially placed to center the focal area in the sample, i.e., so that the center of the ellipsoid coincides with the center of the sample.

[0155] The focal plane of the focal zone will be defined as the transverse plane of the focal zone passing through the center of the ellipsoid. In other words, the maximum cross-section of the ellipsoid is contained in the focal plane.

[0156] Thus, the sample 50 is preferably initially placed so that the focal plane of the focal zone passes through the center of the sample.

[0157] The focusing system 20 defines a multiplicative factor M. This multiplicative factor M is defined as the ratio between the input cross-section (area) of the focusing system and the cross-section (area) of the focal zone in the focal plane. By cross-section of the focal zone in the focal plane is meant that corresponding to the cross-section passing through the center of the ellipsoid.

[0158] The focusing system 20 is arranged and sized to receive all of the acoustic waves emitted by the acoustic wave generators 10.

[0159] The focusing system 20 is configured so as to have a minimum focal length. A minimum focal length advantageously makes it possible to minimize the volume of the focal zone, and therefore to maximize the acoustic intensities achieved in the focal zone. In a preferred embodiment, regardless of the powder used, the focal length is of the order of 100 mm, for a focusing system 20 with a diameter of 15 cm. The focal length increases if the diameter of the focusing system 20 increases.

[0160] The focusing system 20 advantageously has a shape adapted as a function of the acoustic speed of the propagation medium 40 and its own acoustic speed. In an exemplary embodiment of the focusing system 20, said focusing system is a focusing lens. The focusing lens comprises:

[0161] - a first face 21, intended to be opposite the acoustic wave generators 10, having a flat support surface for the acoustic wave generators 10,

[0162] - a second face 22, intended to be opposite the sample 50, having a shape which is a function of the acoustic speed of the propagation medium 40 and the acoustic speed of the focusing lens.

[0163] In this embodiment, the dimensions of the focal zone, and therefore its volume, are thus fixed by the frequency f of the acoustic waves, the speed of propagation of the acoustic waves in the chosen powder, the diameter of the focusing lens and the focal length of the focusing lens.

[0164] By way of non-limiting example, for a sample 50 in the form of graphite powder, for an acoustic wave frequency of 20 MHz, propagation of the acoustic waves in graphite, a focusing system with a diameter of 150 mm and a focal distance of the focusing system of 100 mm, the focal zone is comparable to an ellipsoid, with a circular cross-section, a semi-major axis of 0.5 mm and a semi-minor axis of 0.18 mm.

[0165] In another non-limiting example, for a sample in the form of diamond powder, for an acoustic wave frequency of 20 MHz, propagation of the acoustic waves in the diamond powder, a focusing system with a diameter of 450 mm and a focal distance of the focusing system of 300 mm, the focal zone is comparable to an ellipsoid, with a circular cross-section, a semi-major axis of 1.0 mm and a semi-minor axis of 0.35 mm.

[0166] In the example illustrated in Figure 1, the focusing system 20 is a plano-concave spherical lens. In other words, the first face 21 is planar and the second face 22 is concave.

[0167] The multiplicative factor M of the plano-concave spherical lens is then defined as the ratio between the cross-section (area) of the first face 21 and the cross-section of the focal zone in the focal plane.

[0168] In one embodiment of the device, the focusing system 20 is arranged and held fixedly in a cylinder 19. The barrel 15, comprising the acoustic wave generators 10, is arranged at a first end 191 of the cylinder. A plate, forming a cover 18, is arranged at a second end 192 of the cylinder. The focusing system 20 and the cover 18 are separated by the propagation medium 40.

[0169] In the non-limiting example of figure 1, the cylinder 19 has a circular section.

[0170] Preferably, the propagation medium 40 is chosen so as to promote the propagation of the acoustic waves exiting the focusing system 20 towards the sample 50. The propagation medium 40 is preferably a solid medium or a liquid medium.

[0171] The propagation medium 40 is for example water or liquid gallium.

[0172] Preferably, the focusing system 20 is made of a material having, on the one hand, a propagation speed of the acoustic waves much higher than that of the propagation medium to minimize the focal distance of the focusing system, and thus minimize the dimensions of the focal zone. The focusing system 20 is made of a material having, on the other hand, an acoustic impedance close to the acoustic impedance of the propagation medium 40 to maximize the transmission of the acoustic waves from the focusing system 20 to the propagation medium 40.

[0173] In a preferred embodiment, the focusing system 20 is made of aluminum. The propagation speed of acoustic waves in aluminum is of the order of 6400 m.s' 1 , and that of gallium of 2850 m.s' 1The acoustic impedance of aluminum is of the order of 17 MRayls, just like that of gallium, thus allowing 100% transmission of the acoustic waves from the focusing system 20 to the propagation medium 40.

[0174] The device 100 further comprises a heating system (not shown in the figures) for the sample 50. The heating system is configured to ensure a uniform heating temperature of the sample 50.

[0175] It is clear from the description that the heating system of the sample 50 is an additional element in the device 100 and that the heating system is a separate and independent element from the plurality of acoustic wave generators.

[0176] In a first embodiment, the heating system allows heating by the Joule effect. The heating system comprises, for example, two electric cables, each connected to the container 70 containing the sample 50. The two electric cables are preferably arranged at a distance from each other. The container is traversed by a high-intensity electric current, direct or alternating, with a voltage of a few volts. The applied current is of the order of several kiloamperes. The passage of the electric current within the container causes the sample 50 to heat up.

[0177] Preferably, the container 70 is made of an electrically conductive material, for example graphite.

[0178] With a graphite container, it is possible to heat the sample 50 up to 3000 °C.

[0179] In a second embodiment, the heating system allows heating by inductive effect. Said heating system comprises, for example, an induction coil wound around the container 70. The container is preferably made in the form of a tube. The induction coil is supplied with alternating electric current so as to generate an induced current in the container 70 to heat it and heat the sample 50.

[0180] Preferably, the container 70 is made of an electrically conductive material, for example metal.

[0181] Depending on the metal used, it is possible to heat the sample 50 up to 1000°C- 1500°C.

[0182] As a reminder, diamond is stable at room temperature and pressure but returns to graphite form when heated to more than 800°C in an oxygenated atmosphere and to more than 1700°C in an inert atmosphere or vacuum.

[0183] Preferably, the sample 50, when it is diamond powder, is immersed in an inert atmosphere or under vacuum. By extension, the entire device 100 is immersed in an inert atmosphere or under vacuum.

[0184] In the case of other ceramics, there is no constraint as to the atmosphere in which the sample is immersed.

[0185] In a preferred embodiment, not shown in the figures, the device 100 comprises a member for moving (not shown in the figures) the focal zone in the sample 50. Such a moving member advantageously makes it possible to move the focal zone in the sample to form the ceramic part.

[0186] In one exemplary embodiment of a displacement member, said displacement member is configured to move the sample 50. In another exemplary embodiment of a displacement member, said displacement member is configured to move the focusing system 20.

[0187] In another exemplary embodiment of a displacement member, said displacement member is configured to move the acoustic wave generators 10.

[0188] In another exemplary embodiment of a displacement member, said displacement member is configured to phase shift the acoustic wave generators 10. The first method is now described.

[0189] The first process is based on the principle of high-pressure flash sintering. In other words, the first process allows the ceramic powder to be heated to a predefined temperature, while locally compressing it to a pressure such that the sintering of the ceramic powder can take place.

[0190] It is known to those skilled in the art that the higher the pressure applied, the lower the sintering temperature. As a reminder, the sintering temperature corresponds to the minimum temperature necessary for the sintering of the powder to take place at a given pressure.

[0191] For illustration purposes, Figure 5 shows seven examples of curves illustrating, for different pressure values ​​(in GPa), the minimum temperature required to sinter the powder. Each curve is associated with a type of ceramic powder. In Figure 5, the different powders represented are a diamond powder, a tungsten carbide powder, a silicon nitride powder, a fully stabilized zirconia powder, an alpha alumina powder, a gamma alumina powder, and a titanium oxide powder.

[0192] The first method involves heating the sample 50 to a predetermined temperature.

[0193] The sample is heated until a predetermined minimum temperature is reached. The minimum temperature is preferably higher than the sintering temperature of the chosen ceramic powder, the sintering temperature being a function of the pressure which will be applied at the focal zone of the sample 50.

[0194] As an example, illustrated in Figure 5, for a diamond powder, the sintering temperature is 1400°C for an applied pressure of 4 GPa and 1300°C for an applied pressure of 5 GPa.

[0195] The heating is preferably carried out by the heating system of the device 100. In one embodiment, the heating is carried out by the Joule effect, by passing an electric current through the container 70 via electric cables.

[0196] In another embodiment, the heating is carried out by inductive effect, by generating an induced current in the container 70, via the induction coil.

[0197] Once the sample 50 is at the predetermined minimum temperature, the first method comprises generating and emitting a plurality of acoustic waves.

[0198] It is clear from the description that the step of generating and emitting a plurality of acoustic waves is performed after the heating step. The step of generating and emitting a plurality of acoustic waves is performed only when the sample 50 is at the predetermined minimum temperature.

[0199] Said acoustic waves are emitted simultaneously for predetermined durations, preferably for the same duration.

[0200] As an illustrative example, the acoustic waves are each emitted for a duration of around an hour.

[0201] The generation and emission of the acoustic waves are preferably carried out via the acoustic wave generators 10, controlled by the management unit. The duration of the emission of each of the acoustic waves is controlled by the management unit. Each acoustic wave generator 10 generates and emits an acoustic wave, of predetermined power. Preferably, the powers generated by the acoustic wave generators 10 are identical.

[0202] The acoustic waves together generate at the input of the focusing system 20 an acoustic intensity le.

[0203] As a reminder, the acoustic intensity 1c generated at the input of the focusing system is determined by the ratio between the power injected into the focusing system 20 and the input section of the focusing system 20. The power injected into the focusing system 20 is itself determined by multiplying the number of acoustic wave generators 10 used by the power generated by an acoustic wave generator 10 (based on the principle that all the acoustic wave generators generate the same power).

[0204] The emitted acoustic waves are then focused in the focal zone located in the sample 50. The acoustic waves are emitted towards the focusing system 20, will pass through said focusing system and will all converge towards the sample 50, in the focal zone.

[0205] It is easy to understand that, in the focal zone, the focused acoustic waves will generate an acoustic intensity corresponding, to a factor close to, to the multiplication of the acoustic intensity le at the input of the focusing system by the multiplicative factor M of said focusing system. This multiplicative factor M is a multiplicative factor of the acoustic intensity le, a factor which is induced by the focusing.

[0206] The acoustic intensity is not uniform in the focal zone.

[0207] More precisely, it is known that the spatial distribution of an acoustic intensity in a focal plane is not uniform but appears as an Airy spot. Thus, in the focal plane, the acoustic intensity forms a central (circular) spot and a series of concentric rings around the central spot. The central spot concentrates 85% of the acoustic power. The remaining acoustic power is distributed in a decreasing manner in the series of concentric rings. The average acoustic intensity in the central spot is 0.85 x M x le. The acoustic intensity at 80% of the maximum of the central spot is 2.9 x M x le. The maximum acoustic intensity in the central spot is 3.7 x M x le. This maximum acoustic intensity in the central spot is point-like. The spatial distribution of the acoustic intensity in a plane perpendicular to the focal plane is also not uniform. The acoustic intensity gradually decreases as one moves away from the focal plane.

[0208] The focal zone, for the invention, corresponds, in the focal plane, to the central spot. Thus, since the maximum intensity in the focal zone is maximum in the focal plane, the maximum acoustic intensity in the focal zone is 3.7 x M x le.

[0209] According to the invention, the focused acoustic waves together generate, at least in part of the focal zone, an acoustic intensity at least equal to M x le.

[0210] According to the invention, it is necessary that, at the level of at least one part of the focal zone, the generated acoustic intensity is at least equal to the minimum sufficient acoustic intensity, so that, associated with the heating temperature applied to the sample 50 of ceramic powder, the sintering of the ceramic powder takes place in the at least one part of the focal zone. It is known to those skilled in the art that the acoustic pressure can be expressed as a function of the acoustic intensity. For a given material initially at ambient temperature and pressure, each acoustic intensity value gives a pressure. These acoustic intensity values ​​have been characterized in great detail by the study of the so-called Hugoniot curves, in the book "LASL shock Hugoniot data" published by University of California Press, 1980.

[0211] Thus, knowing the pressure as a function of the acoustic intensity and the minimum temperature necessary for sintering as a function of the pressure for the same powder, it is possible to access the minimum temperature necessary for sintering as a function of the acoustic intensity.

[0212] Figure 6 shows four examples of curves illustrating, for different values ​​of acoustic intensity (in W.rrr 2 ), the minimum temperature required to sinter the powder. Each curve is associated with a type of ceramic powder. In the non-limiting example of Figure 6, the different powders represented are a diamond powder, a tungsten carbide powder, a silicon nitride powder, and an alpha alumina powder.

[0213] Thus, as shown in Figure 6, an acoustic intensity of 4.7.10 11 W.rrr 2applied to a sample 50 of diamond powder heated to a minimum temperature of 1375°C, allows the diamond powder to be sintered, whereas a temperature above 2000°C would be required to sinter the same powder without pressure. Similarly, an acoustic intensity of 4.9.10 10 W.rrr 2 applied to a sample 50 of alpha alumina powder heated to a minimum temperature of 840°C, allows the alpha alumina powder to be sintered, whereas a temperature of 1100°C would be required to sinter this same powder without pressure.

[0214] Depending on the ceramic powder used in the first process, it is possible to associate a pair of values ​​(acoustic intensity, heating temperature) to be applied which advantageously allows the sintering of the powder in at least one part of the focal zone.

[0215] The first method according to the invention allows, from an acoustic intensity of 5.10 11 W.rrr2 , and a minimum heating temperature of 1360°C, to obtain diamond by sintering diamond powder.

[0216] The first method according to the invention makes it possible to obtain diamond by sintering diamond powder, from: - an acoustic intensity of 1.10 11 W.nr 2 , and a minimum heating temperature of 1700°C,

[0217] - with an acoustic intensity of 5.10 13 W.rrr 2 , and a minimum heating temperature of 1000°C.

[0218] Generally, to obtain diamond from diamond powder, the acoustic intensity can vary from 1.10 11 W.rrr 2 at 5.10 13 W.rrr 2 and the minimum heating temperature can vary from 1700°C to 1000°C respectively.

[0219] To prevent the diamond from graphitizing, the minimum heating temperature must not exceed 1700°C, under an inert atmosphere or vacuum.

[0220] Additional examples are described below in a non-limiting manner.

[0221] The first method according to the invention makes it possible to obtain an oxide by sintering oxide powder, from:

[0222] - with an acoustic intensity of 5.10 8 W.rrr 2 , and a minimum heating temperature of 1700°C,

[0223] - with an acoustic intensity of 1.10 12 W.rrr 2 , and a minimum heating temperature of 500°C.

[0224] Generally, to obtain an oxide by sintering oxide powder, the acoustic intensity can vary from 5.10 8 W.rrr 2 at 1.10 12 W.rrr 2 , and a minimum heating temperature can vary from 1700°C to 500°C respectively.

[0225] Among the oxides, we can cite more particularly aluminum oxide or alumina, silicon oxide, titanium oxide, zinc oxide, zirconium oxide or zirconia, molybdenum oxide or even tungsten oxide.

[0226] The first method according to the invention makes it possible to obtain a carbide by sintering carbide powder, from:

[0227] - with an acoustic intensity of 5.10 8 W.rrr 2 , and a minimum heating temperature of 2200°C,

[0228] - with an acoustic intensity of 5.10 12 W.rrr 2 , and a minimum heating temperature of 1000°C.

[0229] Generally, to obtain a carbide by sintering carbide powder, the acoustic intensity can vary from 5.10 8 W.rrr 2 at 5.10 12 W.rrr 2 , and a minimum heating temperature can vary from 2200°C to 1000°C respectively. Tl

[0230] Among the carbides, we can cite more particularly boron carbide, silicon carbide, titanium carbide, zirconium carbide, molybdenum carbide or even tungsten carbide.

[0231] The first method according to the invention makes it possible to obtain a nitride, a boride or a silicide respectively by sintering nitride, boride or silicide powder, from:

[0232] - with an acoustic intensity of 5.10 8 W.rrr 2 , and a minimum heating temperature of 2000°C,

[0233] - with an acoustic intensity of 5.10 12 W.nr 2 , and a minimum heating temperature of 700°C.

[0234] Generally, to obtain a carbide by sintering carbide powder, the acoustic intensity can vary from 5.10 8 W.rrr 2 at 5.10 12 W.rrr 2, and a minimum heating temperature can vary from 2000°C to 700°C respectively.

[0235] Among the carbides, we can cite more particularly boron nitride in its hexagonal form or in its cubic form, aluminum nitride, silicon nitride, titanium nitride, zirconium nitride, molybdenum nitride, tungsten nitride, aluminum boride, titanium boride, zirconium boride or even molybdenum silicide.

[0236] Example 1:

[0237] A simulation was performed to reproduce the device 100 on a small scale and determine a desired volume of the focal area.

[0238] In this simulation, the focusing system 20 is a plano-concave spherical lens. The plano-concave spherical lens has the following dimensions: a diameter of the first face 21 of 7.5 mm, a thickness at its center of 2 mm, and a radius of curvature of the second face 22 of 4 mm. The plano-concave spherical lens is made of aluminum.

[0239] 190 piezoelectric actuators, each 0.5 mm in diameter, are uniformly distributed opposite the first face 21 of the plano-concave spherical lens. The 190 piezoelectric actuators are glued against the first face 21 of the plano-concave spherical lens. Each piezoelectric actuator emits an acoustic wave, with a frequency of 20 MHz. Each piezoelectric actuator generates a power of 1 W. The following values ​​correspond to a sample of diamond powder. For comparison, the values ​​given in parentheses correspond to a sample of alpha alumina powder.

[0240] The simulation gives a focal zone whose volume is comparable to an ellipsoid, with a circular cross-section, semi-major axis 0.92 mm (respectively 0.87 mm) and semi-minor axis 0.33 mm (respectively 0.31 mm).

[0241] From this simulation, we can thus deduce the multiplicative factor M of the plano-concave spherical lens. In the present case, the multiplicative factor M is the cross-section ratio between the entrance section of the plano-concave spherical lens and the cross-section of the focal zone in the focal plane. The multiplicative factor M is thus 1.3.10 2 (respectively 1,4.10 2 ).

[0242] It is known that an increase in the dimensions of the plano-concave spherical lens does not impact the dimensions of the focal zone provided that the same angular aperture is maintained for the plano-concave spherical lens. As a reminder, the angular aperture is proportional to the ratio between the diameter of the first face of the plano-concave spherical lens and the radius of curvature of the second face.

[0243] Thus, a plano-concave spherical lens with a first face of diameter 450 mm (respectively 150 mm) and a second face with a radius of curvature of 240 mm (respectively 80 mm) allows to maintain the dimensions obtained in the simulation for the focal zone. It is known that the thickness of the lens does not impact the angular aperture. The thickness of the lens is fixed at 1 cm to advantageously minimize the travel distance of the acoustic waves while preserving the strength of the lens.

[0244] With such dimensions, the multiplicative factor M of the plano-concave spherical lens is 4.7.10 5 (respectively 5,7.10 4 ).

[0245] To cover the first face of this plano-concave spherical lens, one can use for example 1600 (respectively 150) piezoelectric actuators with a diameter of 10 mm or 10000 (respectively 940) piezoelectric actuators with a diameter of 4 mm or 100 (respectively 10) piezoelectric actuators with a diameter of 40 mm.

[0246] If we choose the example of 1600 (respectively 150) piezoelectric actuators with a diameter of 10 mm and each one generates an acoustic wave with a power of 100 W, the acoustic waves together generate at the input of the lens an acoustic intensity of 1.0.10 6 W.nr 2 (respectively 8,5.10 5 W.rrr 2 ).

[0247] As a reminder, the acoustic intensity corresponds to the ratio between the power injected into the plano-concave spherical lens and the section of the first face of the plano-concave spherical lens. The power injected into the lens corresponds to the multiplication of the number of piezoelectric actuators used by the power generated by each piezoelectric actuator.

[0248] It should be noted that the same acoustic intensity would be obtained at the input of the plano-concave spherical lens with the configuration of 10000 (respectively 940) piezoelectric actuators, of diameter 4 mm, each generating a power of 15.90 W or the configuration of 100 (respectively 10) piezoelectric actuators, of diameter 40 mm, each generating a power of 1590 W. In the configuration with the 1600 (150) piezoelectric actuators, an average acoustic intensity in the focal plane of the focal zone of 4.0.10 is thus obtained. 11 W.rrr2 (corresponding to 0.85 x 4.7.10 5 x 1 ,0.10 6 W.rrr 2 ) (respectively

[0249] 4.1.10 10 W.rrr 2 ) and a maximum acoustic intensity in the focal zone of 1.7.10 12 W.rrr 2 (corresponding to 3.7 x 4.7.10 5 x 1 ,0.10 6 W.rrr 2 ) (respectively

[0250] 1.8.10 11 W.rrr 2 for alpha alumina).

[0251] Thus, in at least part of the focal zone, it is possible to achieve a minimum acoustic intensity of 4.7.10 11 W.rrr 2 (corresponding to 1,6.10 6 x 1 ,0.10 6 W.rrr 2 ) (respectively 4,9.10 10 W.rrr 2 ).

[0252] By heating sample 50, to a minimum temperature of 1375°C (respectively 840°C), in the part of the focal zone where the acoustic intensity reaches 4.7.10 11 W.rrr 2(respectively 4,9.10 10 W.rrr 2 ), the diamond powder (respectively alpha alumina) is densified by sintering.

[0253] The first method according to the invention thus advantageously makes it possible to create sintering of the ceramic powder at the focal zone. The method advantageously makes it possible to finely control the spatial location where the conditions are met to allow the sintering of this ceramic powder.

[0254] In a particular embodiment of the first method, during the emission of acoustic waves, the focal zone is moved in the sample.

[0255] Moving the focal zone in the sample advantageously allows the ceramic powder to be sintered in different areas of the sample until a desired dimension and shape is obtained for the final sintered ceramic part. In an exemplary implementation, to obtain a relative displacement of the focal zone with respect to the sample, the sample is moved by a displacement member.

[0256] In another exemplary implementation, to obtain a relative displacement of the focal zone with respect to the sample, the focusing system is moved by a displacement member.

[0257] In another exemplary implementation, to obtain a relative displacement of the focal zone with respect to the sample, the acoustic wave generators are moved simultaneously by a displacement member.

[0258] In another example of implementation, to obtain a relative displacement of the focal zone with respect to the sample, the acoustic wave generators are phase-shifted with respect to each other with well-determined values ​​to obtain the displacement of the focal zone in the sample.

[0259] The second process is now described. As a reminder, the second process is intended to produce diamond from a sample containing graphite or amorphous carbon powder.

[0260] As a reminder, amorphous carbon contains a mixture of sp-type atomic bonds 2 and sp-type atomic bonds 3 , as well as a possible concentration of hydrogen atoms.

[0261] Advantageously, amorphous carbon, without hydrogen atoms, generally has a sp type hybridization rate 3 between 40% and 90%.

[0262] The second process is advantageously based in a first phase on the principle of dynamic compression, which will be described later, then in a second phase on the principle of high-pressure flash sintering.

[0263] Thus, the second process allows, in a first phase, the conversion of graphite powder into diamond powder, then in a second phase, the densification of the diamond powder by sintering to form a diamond.

[0264] The second process advantageously makes it possible to obtain a dense diamond, therefore a diamond which has improved mechanical resistance.

[0265] The second method involves heating the sample 50 to a predetermined temperature.

[0266] The sample is heated until it reaches a predetermined minimum temperature. The minimum temperature is preferably higher than the sintering temperature of the diamond powder, the sintering temperature being a function of the pressure that will be applied to the sample 50.

[0267] As a reminder, for a diamond powder, the sintering temperature is 1400°C for an applied pressure of 5 GPa.

[0268] Heating is preferably carried out by the heating system of the device 100.

[0269] In one embodiment, the heating is carried out by the Joule effect, by passing an electric current through the container 70 via electric cables.

[0270] In another embodiment, the heating is carried out by inductive effect, by generating an induced current in the container 70, via the induction coil.

[0271] Once the sample is at the predetermined minimum temperature, the second method involves generating and emitting a plurality of acoustic waves.

[0272] It is clear from the description that the step of generating and emitting a plurality of acoustic waves is performed after the heating step. The step of generating and emitting a plurality of acoustic waves is initiated only when the sample 50 is at the predetermined minimum temperature.

[0273] Said acoustic waves are emitted simultaneously for predetermined durations, preferably for the same duration.

[0274] As an illustrative example, the acoustic waves are each emitted for a duration of around an hour.

[0275] The generation and emission of the acoustic waves are preferably carried out via the acoustic wave generators 10, controlled by the management unit. The duration of the emission of each of the acoustic waves is controlled by the management unit. Each acoustic wave generator 10 generates and emits an acoustic wave, of predetermined power. Preferably, the powers generated by the acoustic wave generators 10 are identical.

[0276] The acoustic waves together generate at the input of the focusing system 20 an acoustic intensity le.

[0277] As for the first method, the acoustic intensity 1e generated at the input of the focusing system is determined by the ratio between the power injected into the focusing system 20 and the input section of the focusing system 20. The power injected into the focusing system 20 is itself determined by multiplying the number of acoustic wave generators 10 used by the power generated by an acoustic wave generator 10 (based on the principle that all the acoustic wave generators generate the same power).

[0278] The emitted acoustic waves are then focused in the focal zone located in the sample 50. The acoustic waves are emitted towards the focusing system 20, will pass through said focusing system and will all converge towards the sample 50, in the focal zone.

[0279] As for the first method, in the focal zone, the focused acoustic waves will generate an acoustic intensity corresponding, to a factor close to, to the multiplication of the acoustic intensity le at the input of the focusing system by the multiplicative factor M of said focusing system. This multiplicative factor M is a multiplicative factor of the acoustic intensity le, a factor which is induced by the focusing.

[0280] As in the first method, in the focal zone, the acoustic intensity is not uniform in the focal zone. More precisely, it is known that the spatial distribution of an acoustic intensity in a focal plane is not uniform but appears as an Airy spot. Thus, in the focal plane, the acoustic intensity forms a central (circular) spot and a series of concentric rings around the central spot. The central spot concentrates 85% of the acoustic power. The remaining acoustic power is distributed in a decreasing manner in the series of concentric rings. The average acoustic intensity in the central spot is 0.85 x M x le. The acoustic intensity at 80% of the maximum of the central spot is 2.9 x M x le. The maximum acoustic intensity in the central spot is 3.7 x M x le. This maximum acoustic intensity in the central spot is punctate.The spatial distribution of acoustic intensity in a plane perpendicular to the focal plane is also not uniform. The acoustic intensity gradually decreases as it moves away from the focal plane.

[0281] The focal zone, for the invention, corresponds, in the focal plane, to the central spot. Thus, since the maximum intensity in the focal zone is maximum in the focal plane, the maximum acoustic intensity in the focal zone is 3.7 x M x le.

[0282] According to the invention, for a graphite powder, the focused acoustic waves must together generate, at least in part of the focal zone, an acoustic intensity at least equal to 1.10 12 W.rrr 2 , intensity corresponding to a minimum acoustic intensity sufficient to achieve a phase change from graphite to diamond. According to the invention, for an amorphous carbon powder having a sp-type hybridization rate 3between 40% and 90%, the focused acoustic waves must together generate, at least in part of the focal zone, an acoustic intensity at least equal to 2.1.10 12 W.rrr 2 , intensity corresponding to a minimum acoustic intensity sufficient to achieve a phase change from amorphous carbon to diamond.

[0283] More precisely, for an amorphous carbon powder exhibiting a sp-type hybridization rate 3 of 40%, the focused acoustic waves must together generate, at least in part of the focal zone, an acoustic intensity at least equal to 1.4.10 12 W.nr 2 , intensity corresponding to a minimum acoustic intensity sufficient to achieve a phase change of said amorphous carbon into diamond. For an amorphous carbon powder having a sp-type hybridization rate 3of 90%, the focused acoustic waves must together generate, at least in part of the focal zone, an acoustic intensity at least equal to 2.1 .10 12 W.rrr 2 , intensity corresponding to a minimum acoustic intensity sufficient to achieve a phase change of said amorphous carbon into diamond.

[0284] The following description and values ​​are given using the example of a graphite powder sample. To avoid repetition of the description for an amorphous carbon powder sample, the values ​​associated with an amorphous carbon powder sample will be given in parentheses.

[0285] To obtain an acoustic intensity of 1.10 12 W.rrr 2 (respectively between 1, 4.10 12 W.rrr 2 and 2.1 .10 12 W.rrr 2 according to the sp-type hybridization rate 3) at least in part of the focal zone, it is possible to act on both the acoustic intensity l e generated at the input of the focusing system and on the multiplicative factor M of the focusing system. It is possible to influence several parameters in the device, such as the number of acoustic wave generators, the power generated by these acoustic wave generators, the input section of the focusing system, the multiplicative factor M of the focusing system, to achieve an acoustic intensity of 1.10 12 W.rrr 2 (respectively between 1, 4.10 12 W.rrr 2 and 2, 1, 10 12 W.rrr 2 according to the sp-type hybridization rate 3) at least in part of the focal zone. As previously stated, the second method according to the invention advantageously relies on the principle of dynamic compression to convert graphite (respectively amorphous carbon) into diamond.

[0286] To achieve the phase change of graphite (respectively amorphous carbon) into diamond, by dynamic compression, it is known to those skilled in the art that two conditions must be met:

[0287] - be at sufficient pressures so that the diamond is more stable than graphite (respectively amorphous carbon),

[0288] - confer sufficient kinetic energy to the carbon atoms to break the metastability of graphite (respectively of amorphous carbon) induced by the double bonds between carbon atoms.

[0289] In other words, sufficient pressure on the graphite (respectively the amorphous carbon) will make the diamond stable and sufficient kinetic energy will allow the phase change to occur.

[0290] For dynamic compression using shock waves, as is the case in the prior art, the pressure and kinetic energy are provided by the shocks. For a shock wave, the pressure and kinetic energy can be expressed as a function of the shock intensity. The shock intensity must then be sufficient so that:

[0291] - the pressure reached exceeds a sufficient minimum pressure making the diamond more stable than graphite (respectively amorphous carbon), for example at least 5 GPa; 5 GPa corresponding to the stability pressure of diamond in static compression;

[0292] - the kinetic energy of the atoms is sufficient to break the metastability of said graphite (respectively of said amorphous carbon) during the duration of the emission of the shock waves.

[0293] For example, in the publication by Erskine et al., it was shown that, for dynamic compression carried out using shock waves, the conversion of a graphite sample into diamond occurs in a few hundred nanoseconds from 6 GPa and is complete in less than 10 ns from 20 GPa.

[0294] Similarly, for compression with acoustic waves the pressure and kinetic energy can be expressed as a function of the acoustic intensity.

[0295] The acoustic intensity must be sufficient so that: - the pressure reached exceeds a sufficient minimum pressure making the diamond more stable than graphite (respectively amorphous carbon),

[0296] - the kinetic energy of the atoms is sufficient to break the metastability of said graphite (respectively of said amorphous carbon) during the duration of the emission of the acoustic waves.

[0297] For a given material initially at room temperature and pressure, each acoustic intensity value gives a unique pair of values ​​(pressure, kinetic energy). These acoustic intensity values ​​have been characterized in great detail by the study of so-called Hugoniot curves, in the book "LASL shock Hugoniot data" published by University of California Press, 1980.

[0298] The effect of kinetic energy on the conversion of graphite (respectively amorphous carbon) into diamond can be modeled by a "kinetic" Arrhenius law, similar to the effect of temperature. This modeling accounts for the role played by kinetic energy in a dynamic compression, replacing thermal energy in a static compression, to enable the transition of graphite (respectively amorphous carbon) into diamond. For example, a dynamic compression in graphite (respectively amorphous carbon with a sp hybridization rate 3 between 40% and 90%) with an acoustic intensity of around 2.10 13 W.rrr 2 (respectively 2.7.10 13 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 4.0.10 13 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3of 90%)) induces a pressure of 25 GPa (respectively 34 GPa (for amorphous carbon powder with a sp-type hybridization rate 3 of 40%) at 49 GPa (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)) and a kinetic energy of 0.16 eV, a value equivalent to a kinetic energy of thermal origin induced by a temperature of the order of 1500°C.

[0299] Figure 7 represents a modeling curve illustrating the induced pressure as a function of the acoustic intensity for a graphite powder (solid line curve), an amorphous carbon powder with a sp-type hybridization rate 3 of 40% (dashed curve) and an amorphous carbon powder with an sp-type hybridization rate 3 of 90% (dotted curve).

[0300] Figure 8 represents a modeling curve illustrating the kinetic energy as a function of the acoustic intensity for a graphite powder (solid line curve), an amorphous carbon powder with a sp-type hybridization rate 3 of 40% (dashed curve) and an amorphous carbon powder with an sp-type hybridization rate 3of 90% (dotted curve). When the acoustic intensity is sufficient, the duration of the acoustic intensity emission then conditions the proportion of the phase change, i.e. the percentage of conversion of graphite (respectively amorphous carbon) into diamond. In other words, for a given emitted acoustic intensity, the percentage of conversion of graphite (respectively amorphous carbon) into diamond is a function of the duration of the emission of this acoustic intensity. More precisely, the greater the duration of the acoustic intensity emission, the greater the percentage of conversion of graphite (respectively amorphous carbon) into diamond. Moreover, for a desired percentage of conversion of graphite (respectively amorphous carbon) into diamond, the greater the emitted acoustic intensity, the shorter the duration of the acoustic intensity emission.

[0301] Based on Arrhenius's law, the reaction rate k can be approximated by the following relation:

[0302] -Eg k = A. exp~ E neck

[0303] E a is the activation energy of the conversion of graphite into diamond, i.e. the energy required to supply the carbon atoms to break the metastability of graphite, and

[0304] A is a constant depending essentially on the mechanism involved for the phase change (thermal diffusion, martensitic, etc.).

[0305] E a is of the order of 0.2 eV, according to the publication of Xie et al, 2017, Journal of the American Chemical Society, 139, 7, pages 2545-2548

[0306] From the results of past experiments by Erskine et al., the constant A is of the order of 2.3.10 9 s -1 .

[0307] Thus, for a dynamic compression of a graphite powder (respectively an amorphous carbon powder with a sp-type hybridization rate 3 between 40% and 90%) with an acoustic intensity of the order of 1.1.10 12 W.rrr 2 (respectively 1,4.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 2.1 .10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)), a pressure of the order of 5 GPa (corresponding to the stability pressure of diamond) (respectively of 6.5 GPa (for amorphous carbon powder with a sp-type hybridization rate 3 of 40%) at 9.5 GPa (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)) and a kinetic energy of the order of 0.012 eV (identical value for amorphous carbon with a sp type hybridization rate 3between 40% and 90%) can be obtained. This kinetic energy induces a phase change rate k of the order of 410 s _1 (identical value for amorphous carbon with sp-type hybridization rate 3 between 40% and 90%).

[0308] The percentage of phase change is expressed in

[0309] 1 — exp~ kt where t is time.

[0310] Thus, we have a conversion of 95% in 3 / k seconds, a conversion of 99% in 5 / k seconds. It is thus possible to achieve a conversion of 99% of the zone under dynamic compression in a time of the order of 0.01 seconds (identical value for amorphous carbon with a sp type hybridization rate 3 between 40% and 90%).

[0311] For illustrative purposes, Figure 9 represents a graph illustrating the duration required for the emission of an acoustic wave as a function of the intensity of said acoustic wave to recreate the conditions sufficient to achieve a 99% phase change of a graphite sample into diamond (continuous line curve), of an amorphous carbon sample with a sp-type hybridization rate 3 between 40% diamond (dashed curve) and an amorphous carbon sample with a sp hybridization rate 3 between 40% in diamond (dotted curve).

[0312] Thus, as shown in Figure 9, for a graphite powder, an acoustic intensity of the order of 1.4.10 13 W.nr 2allowing a pressure of 20 GPa to be reached, a pressure higher than the minimum pressure sufficient for the diamond to be stable, gives the carbon atoms a kinetic energy such that at least 99% of the graphite sample will be converted into diamond in a time of the order of 10 ns. An acoustic intensity of the order of 1.6.10 12 W.rrr 2 , allowing, for its part, to reach a pressure of 6 GPa, pressure higher than the minimum pressure sufficient for the diamond to be stable, gives the carbon atoms a kinetic energy of 0.017 eV and such that at least 99% of the graphite sample will be converted into diamond in a duration of the order of 1 ms. On the other hand, an acoustic intensity lower than 5.10 11 W.rrr 2 will induce a phase change in a period greater than a hundred hours provided that the pressure is always sufficient to be in the diamond stability regime.

[0313] Returning to the second method according to the invention, the focused acoustic waves will thus generate at the focal zone, an acoustic intensity inducing an acoustic pressure and kinetic energy, as described previously.

[0314] For the phase change of graphite (respectively amorphous carbon with sp-type hybridization rate 3 between 40% and 90%) in diamond takes place in the focal zone, it is necessary that, at least in part of the focal zone, the acoustic intensity generated is at least equal to the minimum sufficient acoustic intensity, i.e. 1.10 12 W.rrr 2 (respectively 1,4.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 2.1 .10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)).

[0315] As described above, when the acoustic intensity obtained at the focal zone is sufficient to achieve a phase change of graphite (respectively amorphous carbon) into diamond, the duration of the emission of said acoustic intensity will condition the proportion of the phase change of graphite (respectively amorphous carbon) into diamond in the focal zone. In other words, for a given acoustic intensity at the focal zone, the percentage of conversion of graphite (respectively amorphous carbon) into diamond, in the focal zone, is a function of the duration of the emission of the acoustic waves. More precisely, for a given acoustic intensity, the percentage of conversion of graphite (respectively amorphous carbon) into diamond increases with the duration of the emission of the acoustic waves.

[0316] The second method, in a second phase, once the conversion of graphite (respectively amorphous carbon) into diamond powder or very low density diamond, by combining the pressure with the heating temperature of the sample 50, the conditions are also advantageously met to carry out the sintering in order to densify the diamond. The second method according to the invention allows, from an acoustic intensity of 1.10 12 W.nr 2 (respectively 1,4.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 2, 1.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)), and a minimum heating temperature of 1300°C (respectively 1250°C (for amorphous carbon powder with sp-type hybridization rate 3of 40%) at 1100°C (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)), to obtain diamond from graphite powder (respectively amorphous carbon with a sp-type hybridization rate 3 between 40% and 90%), by conversion of graphite (respectively amorphous carbon with a sp-type hybridization rate 3 between 40% and 90%) in diamond and sintering.

[0317] To prevent the diamond from graphitizing, the minimum heating temperature must not exceed 1700°C, under an inert atmosphere or vacuum.

[0318] Example 2:

[0319] A simulation was performed to reproduce the device 100 on a small scale and determine a desired volume of the focal area.

[0320] In this simulation, the focusing system 20 is a plano-concave spherical lens. The plano-concave spherical lens has the following dimensions: a diameter of the first face 21 of 7.5 mm, a thickness at its center of 2 mm, and a radius of curvature of the second face 22 of 4 mm. The plano-concave spherical lens is made of aluminum.

[0321] 190 piezoelectric actuators, each with a diameter of 0.5 mm, are uniformly distributed opposite the first face 21 of the plano-concave spherical lens. The 190 piezoelectric actuators are glued against the first face 21 of the plano-concave spherical lens. Each piezoelectric actuator emits an acoustic wave, with a frequency of 20 MHz. Each piezoelectric actuator generates a power of 1 W.

[0322] The following values ​​correspond to a graphite powder sample. For comparison, the values ​​given in parentheses correspond to an amorphous carbon powder sample with a sp hybridization rate. 3 between 40% and 90%.

[0323] The simulation gives a focal zone whose volume is comparable to an ellipsoid, with a circular cross-section, with a semi-major axis of 0.5 mm (respectively 0.66 (for amorphous carbon powder with a sp-type hybridization rate 3 from 40%) to 0.87 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)) and semi-minor axis 0.18 mm (respectively 0.24 mm (for amorphous carbon powder with sp-type hybridization rate 3 40%) to 0.32 mm (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)).

[0324] From this simulation, we can thus deduce the multiplicative factor M of the plano-concave spherical lens. In the present case, the multiplicative factor M is the cross-sectional ratio between the entrance section of the plano-concave spherical lens and the cross-sectional area of ​​the focal zone in the focal plane. The multiplicative factor M is thus 4.3.10 2 (respectively 2.4.10 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 1,3.10 2 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)).

[0325] It is known that an increase in the dimensions of the plano-concave spherical lens does not impact the dimensions of the focal zone provided that the same angular aperture is maintained for the plano-concave spherical lens. As a reminder, the angular aperture is proportional to the ratio between the diameter of the first face of the plano-concave spherical lens and the radius of curvature of the second face.

[0326] Thus, a plano-concave spherical lens with a first face of diameter 450 mm and a second face with a radius of curvature of 240 mm allows to maintain the dimensions obtained in the simulation for the focal zone. It is known that the thickness of the lens does not impact the angular aperture. The thickness of the lens is fixed at 1 cm to advantageously minimize the travel distance of the acoustic waves while preserving the strength of the lens.

[0327] With such dimensions, the multiplicative factor M of the plano-concave spherical lens is 1.6.10 6 (respectively 8, 9.10 5 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 4.7.10 5 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)).

[0328] To cover the first face of this plano-concave spherical lens, we can use for example 1600 piezoelectric actuators with a diameter of 10 mm or 10000 piezoelectric actuators with a diameter of 4 mm or 100 piezoelectric actuators with a diameter of 40 mm.

[0329] If we choose the example of 1600 piezoelectric actuators with a diameter of 10 mm and each one generates an acoustic wave with a power of 100 W (respectively 200 W), the acoustic waves together generate at the input of the lens an acoustic intensity of 1.0.10 6 W.nr 2(respectively 2.0.10 6 W.rrr 2 ).

[0330] As a reminder, the acoustic intensity corresponds to the ratio between the power injected into the plano-concave spherical lens and the section of the first face of the plano-concave spherical lens. The power injected into the lens corresponds to the multiplication of the number of piezoelectric actuators used by the power generated by each piezoelectric actuator.

[0331] It should be noted that the same acoustic intensity would be obtained at the input of the plano-concave spherical lens with the configuration of 10,000 piezoelectric actuators (4 mm diameter) each generating a power of 15.90 W or the configuration of 100 piezoelectric actuators (40 mm diameter) each generating a power of 1590 W.

[0332] In the configuration with 1600 piezoelectric actuators, an average acoustic intensity in the focal plane of the focal zone of 1.3.10 is thus obtained 12 W.rrr 2 (0.85 x 1.6.10 6 x 1 ,0.10 6 W.rrr 2 ) (respectively of 1,5.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 0.8.10 12 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)) and a maximum acoustic intensity in the focal zone of 5.4.10 12 W.rrr 2 (3.7 x 1.6.10 6 x 1 ,0.10 6 W.rrr 2 ) (respectively 6.6.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to

[0333] 3.6.10 12 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)).

[0334] Thus, in a part of the focal zone, it is possible to reach an acoustic intensity greater than 10 12 W.rrr 2 (respectively 1,4.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to

[0335] 2.1 .10 12 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)), acoustic intensity which is sufficient to produce diamond powder from graphite powder (respectively an amorphous carbon powder a sp-type hybridization rate 3 between 40% and 90%).

[0336] By heating the sample 50 in parallel, to a minimum temperature of 1300°C (respectively 1250°C (for the amorphous carbon powder with a sp-type hybridization rate 3 of 40%) at 1100°C (for amorphous carbon powder with sp-type hybridization rate 3of 90%)), in the part of the focal zone where the acoustic intensity reaches 1.10 12 W.nr 2 (respectively 1,4.10 12 W.rrr 2 (for amorphous carbon powder with sp-type hybridization rate 3 from 40%) to 2.1, 10 12 (for amorphous carbon powder with sp-type hybridization rate 3 of 90%)), we thus densify, by sintering, the diamond powder obtained.

[0337] The second method according to the invention thus advantageously makes it possible to create a diamond seed, or to enlarge a seed previously present in the sample, at the level of the focal zone.

[0338] The second method advantageously allows for fine control of the spatial location where the conditions are met to allow the formation of diamond. The second method according to the invention also advantageously allows for the production of diamond, without destroying the sample, unlike shock wave technologies.

[0339] The second method according to the invention also makes it possible to control the duration of the emission of acoustic waves.

[0340] In a particular embodiment of the second method, during the emission of acoustic waves, the focal zone is moved in the sample.

[0341] This particular mode of the second method is implemented when the acoustic waves are emitted for a duration greater than the emission duration necessary to convert a desired predefined percentage of graphite (respectively amorphous carbon) into diamond in the focal zone.

[0342] Moving the focal zone in the sample advantageously allows the diamond seed to grow until a desired size and shape is obtained for the diamond. The direction of movement of the focal zone in the sample allows the direction of diamond growth to be directed to obtain the desired shape.

[0343] The speed of movement of the focal zone in the sample is advantageously related to the duration of the emission of the acoustic waves necessary to convert a desired predefined percentage of graphite (respectively amorphous carbon) into diamond in the focal zone. Therefore the speed of movement of the focal zone in the sample is a function of the acoustic intensity of the acoustic wave in the focal zone, because the acoustic intensity determines the duration of the emission necessary to enable the conversion of the desired predefined percentage of graphite (respectively amorphous carbon) into diamond.

[0344] Preferably, since the duration of the emission necessary to allow the conversion of all or part of the graphite (respectively of the amorphous carbon) into diamond, is very short, generally less than a second, the movement of the focal zone can be carried out continuously, by adapting the speed of movement of the focal zone.

[0345] In an exemplary implementation, to obtain a relative displacement of the focal zone with respect to the sample, the sample is moved by a displacement member.

[0346] In another exemplary implementation, to obtain a relative displacement of the focal zone with respect to the sample, the focusing system is moved by a displacement member.

[0347] In another exemplary implementation, to obtain a relative displacement of the focal zone with respect to the sample, the acoustic wave generators are moved simultaneously by a displacement member.

[0348] In another example of implementation, to obtain a relative displacement of the focal zone with respect to the sample, the acoustic wave generators are phase-shifted with respect to each other with well-determined values ​​to obtain the displacement of the focal zone in the sample.

[0349] The above description clearly illustrates that through its various characteristics and their advantages, the present invention achieves the objectives it set for itself. In particular, it provides two methods for producing diamond which have the advantage of not destroying the sample, and of finely controlling the direction of growth of the diamond.

Claims

Claims Claim 1. Method for producing a ceramic part from a sample (50) of ceramic powder, comprising: - heating, by a heating system, the sample (50) of ceramic powder to a predetermined minimum temperature, then - the generation and emission, by a plurality of acoustic wave generators, of a plurality of acoustic waves, towards a focusing system (20), each acoustic wave generator being configured to emit an acoustic wave of frequency f between 1 MHz and 100 MHz, said acoustic waves together generating at the input of the focusing system an acoustic intensity I e , - focusing said acoustic waves, by the focusing system (20), into a focal zone in the sample (50) of ceramic powder, the focusing system defining a multiplicative factor M, said focused acoustic waves generating together, at at least a part of the focal zone, a minimum acoustic intensity of at least M x le, sufficient to, combined with the minimum heating temperature applied to the sample (50) of ceramic powder, sinter the ceramic powder in said part of the focal zone. Claim 2. The method of claim 1 wherein the minimum heating temperature of the sample (50) of ceramic powder is greater than a sintering temperature of the ceramic powder. Claim 3. Method according to one of the preceding claims in which the ceramic powder is chosen from: a diamond powder, a tungsten carbide powder, a silicon nitride powder, a fully stabilized zirconia powder, an alpha alumina powder, a gamma alumina powder, and a titanium oxide powder. Claim 4. Method according to one of the preceding claims comprising, during the emission of the acoustic waves, the displacement of the focal zone in the sample (50). Claim 5. Device (100) for implementing the method according to one of the preceding claims comprising: - a sample heating system (50), - a plurality of acoustic wave generators (10), each configured to generate an acoustic wave of frequency f between 1 MHz and 100 MHz, the plurality of acoustic wave generators (10) and the sample heating system (50) being separate elements in the device (100), - a focusing system (20) of the acoustic waves into a focal zone in the sample (50), the focusing system defining the multiplicative factor M. Claim s. Device (100) according to claim 4 in which each acoustic wave generator (10) is a piezoelectric actuator. Claim 7. Device (100) according to one of claims 4 or 5 comprising a member for moving the focal zone in the sample (50). Claim 8. Device (100) according to the preceding claim in which the member for moving the focal zone in the sample (50) is a member for moving the acoustic wave focusing system and / or a member for moving the sample and / or a member for moving the acoustic wave generators and / or a member for phase shifting the acoustic wave generators. Claim 9. A method for producing diamond from a sample (50) comprising graphite powder, comprising: - heating, by a heating system, the sample (50) of graphite powder to a predetermined minimum temperature, then - the generation and emission, by a plurality of acoustic wave generators, of a plurality of acoustic waves, towards a focusing system (20), each acoustic wave generator being configured to emit an acoustic wave of frequency f between 1 MHz and 100 MHz, said acoustic waves together generating at the input of the focusing system an acoustic intensity / e , - focusing said acoustic waves, by the focusing system (20), into a focal zone in the graphite powder sample (50), the focusing system defining a multiplicative factor M, said focused acoustic waves generating together, at least in part of the focal zone, an acoustic intensity at least equal to M x le = 1.10 12 W.nr 2corresponding to a minimum acoustic intensity sufficient to: o in a first phase, carry out a phase change of the graphite powder into diamond powder, then o in a second phase, combined with the minimum heating temperature applied to the sample (50), sinter the diamond powder obtained. Claim 10. The method of claim 9 wherein the minimum heating temperature of the sample (50) comprising graphite powder is greater than a sintering temperature of the graphite powder. Claim 11. A method for producing diamond from a sample (50) comprising an amorphous carbon powder, comprising: - heating, by a heating system, the sample (50) of amorphous carbon powder to a predetermined minimum temperature, then - the generation and emission, by a plurality of acoustic wave generators, of a plurality of acoustic waves, towards a focusing system (20), each acoustic wave generator being configured to emit an acoustic wave of frequency f between 1 MHz and 100 MHz, said acoustic waves together generating at the input of the focusing system an acoustic intensity I e , - focusing said acoustic waves, by the focusing system (20), into a focal zone in the sample (50) of amorphous carbon powder, the focusing system defining a multiplicative factor M, said focused acoustic waves generating together, at at least part of the focal zone, an acoustic intensity at least equal to M x le = 2.1, 10 12 W.rrr 2 corresponding to a minimum acoustic intensity sufficient for: o in a first phase, carry out a phase change of the amorphous carbon powder into diamond powder, then o in a second phase, combined with the minimum heating temperature applied to the sample (50), sinter the diamond powder obtained. Claim 12. The method of claim 11 wherein the minimum heating temperature of the sample (50) comprising an amorphous carbon powder is greater than a sintering temperature of the amorphous carbon powder.

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