Mould for a sintering device assisted by pulsed electric current

Abstract

The invention relates to a mould (120) for use in an SPS sintering device for consolidating a sample (20) comprising a substrate and a stack of layers (22) printed at the centre of the substrate, wherein the SPS device comprises two movable pistons adapted to apply a uniaxial force along a vertical axis A to the mould in order to consolidate the stack of layers. The mould (120) is configured such that the sample (20) is held between two planar internal faces (134, 144) and extends in a plane containing the vertical axis A, and when the two movable pistons move toward each other in order to apply a vertical uniaxial force FV along the axis A, part of the uniaxial force FV applied to the mould is decomposed into a tangential component Ft and a normal component FN, wherein the normal component is capable of generating a horizontal compressive force FH in order to consolidate the stack of layers.

Description

Description Title: Mold for pulsed electric current assisted sintering device Technical field

[0001] The present disclosure relates to the field of densification or consolidation of an assembly of materials by hot sintering with pulsed electric current. It relates more particularly to a mold intended to be used in a hot sintering device with pulsed electric current ("Spark Plasma Sintering" or SPS in English) which makes it possible to consolidate a sample having a longitudinal shape, under a controlled atmosphere, and making it possible to work between an ambient temperature (25°C), and a high temperature, for example greater than 1000°C and up to 2500°C.

[0002] The present disclosure also relates to an SPS sintering device comprising this mold forming a sintering chamber.

[0003] The present disclosure finally relates to a controlled atmosphere SPS sintering process which uses such an SPS apparatus. Prior art

[0004] The "Spark Plasma Sintering" (SPS) sintering technique, also called hot sintering with pulsed electric field, or sintering assisted by pulsed electric current, is a consolidation process which allows, through densification, the shaping and assembly of a wide range of materials such as metals, ceramics, polymers, and composite materials.

[0005] The technique consists of simultaneously applying uniaxial pressure and high intensity electric current pulses to a sample to be densified, or to parts to be assembled, which cause a rise in temperature within the matrix.

[0006] Figure 1 schematically represents a known SPS sintering device 1.

[0007] This SPS device comprises a matrix 2, an upper piston 3, a lower piston 4, an upper electrode 7, a lower electrode 8, a current pulse train generator 6.

[0008] The die is a hollow cylindrical body 2 in which a chamber is defined for receiving a material to be densified such as a ceramic powder. The upper piston 3 and the lower piston 4 are moved towards each other inside the chamber for the application of a uniaxial force along a vertical axis A on the powder to be densified. Each piston has a bearing surface placed at its free end intended to come into contact with the powder to be densified. The chamber is therefore defined by the free end of the upper piston, the free end of the lower piston and the die.

[0009] The upper electrode 7 and the lower electrode 8 allow the upper and lower pistons to be connected respectively to the pulsed current generator 6.

[0010] The device 1 also comprises spacers 9, 10 interposed between the electrode and the piston. In the example shown in FIG. 1, the device comprises two upper spacers interposed between the upper electrode and the end of the upper piston opposite its bearing surface, two spacers interposed between the lower electrode and the end of the lower piston opposite its bearing surface.

[0011] The electric current is therefore applied to the powder contained in the chamber via an assembly of pistons and spacers.

[0012] The device also comprises one or more temperature probes 5, such as thermocouples, which make it possible to control and monitor the sintering temperature with a view to regulating it by control electronics (not shown). As indicated above, the device makes it possible to reach very high temperatures, up to 2500°C, very quickly with a temperature rise rate of up to 500°C / minute.

[0013] The die, pistons, spacers and electrodes are placed inside a vacuum or atmosphere-controlled enclosure 12.

[0014] The entire tooling is connected in series, from the upper electrode to the lower one located at the ends of the equipment.

[0015] The die, pistons, and spacers are typically made of graphite and provide Joule heating to the sample loaded in the middle of the die. The inner wall of the die is covered with graphite sheets to limit any reaction with the sample and also to facilitate demolding.

[0016] The operating principle of the device is described below with reference to Figure 1.

[0017] A very high pressure and current are applied between the upper and lower electrodes. The pressure and current are transmitted to the upper and lower pistons via upper and lower spacers or plates. The very high direct current passes through the spacers, pistons and the sample in the form of successive pulses at a defined frequency, which allows a very rapid temperature rise and complete sintering in a few minutes. This technique allows in particular to control the microstructure of the material down to nanometric scales.

[0018] The current is applied in the form of trains of current pulses, for example with a period of a few milliseconds, the intensity of which can reach several thousand amperes, for example up to 5000 A. Its intensity depends on the equipment used.

[0019] The material to be densified in the chamber can be powder as illustrated in figure 1, a ceramic part to be densified, or two parts for example in ceramic to be assembled to obtain a stack of parts.

[0020] The particularity of SPS technology lies in the fact that the heat source is not external but that an electric current applied via the electrodes passes through the press die and also the sample, in powder form, when it is conductive.

[0021] Thus, the die, spacers and pistons act as a heating source, which allows for high heating rates. Unlike conventional sintering, the sample is heated from the outside to the inside by thermal conduction from the die.

[0022] The SPS technique therefore offers numerous technological and economic advantages over conventional sintering processes. One of the main advantages of the SPS technique is the ability to densify samples, even highly refractory ones such as ceramics, in very short times of around a few tens of minutes. The speed of sintering thus makes it possible to limit granular growth and to achieve a density close to 100% for certain materials, thus improving the mechanical properties of the densified materials.

[0023] One of the major constraints of the SPS process comes from the design of the SPS equipment, and in particular from the shape and size of the die. Indeed, the geometric shape and its size have a direct influence on the two main operating parameters of the device, which are temperature and pressure.

[0024] Thus, a less powerful generator will limit the maximum temperature for large molds and a piston with a smaller application surface will limit the pressure applied to the sample that has large diameters. In addition, when starting the device, it is necessary to apply a minimum force on the sample to ensure safe operation and this minimum force is applied almost instantaneously. Also, the mold must have a minimum size to withstand this minimum force.

[0025] For example, in the case of an SPS device with a 30 kW generator and a pressure force of 50 kN (5 Tonnes) which applies a minimum force of 3 kN (i.e. a pressure of 40 MPa for a diameter of 10 mm), the experimental results show that large molds, i.e. having a diameter equal to or greater than 20 mm, limit the maximum working temperature and the maximum pressure.

[0026] Another constraint of the SPS process comes from the design of the mold, the geometric shape and dimensions of which are reserved for the manufacture of small parts, generally having a diameter between 10 and 20 mm and a thickness between 2 and 100 mm, or even 1000 mm, with a geometric cylinder shape.

[0027] In the context of the present invention and as illustrated in FIG. 2, the sample to be sintered 20 has an elongated and non-cylindrical shape. It comprises a substrate 21 having a width Li of between 2 and 3 mm and a length L2 of between 30 and 40 mm. The substrate has a thickness generally of between 100 and 250 μm. It also comprises a stack of printed layers 22 to be co-sintered located in the center of the substrate. The stack of layers to be co-sintered, on the contrary, has a dimension of between 50 and 600 μm and comprises a functional pattern which must be preserved after the co-sintering cycle.

[0028] Using a conventional mold to co-sinter such a sample would require a mold with an internal diameter between 30 and 40 mm. Such a mold would involve significant energy consumption to achieve the desired working temperature with difficulty, or not at all. In addition, experimental results have shown that the temperature distribution along the radial direction of the sample is less homogeneous as the sample diameter increases. In other words, this temperature gradient can lead to a densification gradient from the center to the edge of the sample.

[0029] Using a conventional mold would imply that the uniaxial force applied by the pistons is applied directly to the stack of layers to be co-sintered. Consequently, the effect of applying the minimum force suddenly to the stack of layers can lead to partial or total destruction of the printed pattern.

[0030] It is apparent from the above that there is a need for a mold for use in a standard SPS device, and an SPS method using this device, which makes it possible to reduce and control the force to be applied to a central area of ​​the sample having a dimension close to the minimum force applied by the device, while controlling the support between the layers during the sintering cycle to ensure the cohesion of the layers, without degrading or damaging the printed pattern of the central stack of layers.

[0031] Another object of the present disclosure is to provide a mold with a specific geometric shape which makes it possible to reduce the volume of material to be heated in order to reduce energy consumption compared to a conventional mold, and to ensure a homogeneous temperature distribution when used in an SPS device. Summary

[0032] This disclosure improves the situation.

[0033] A mold is provided for use in a hot sintering device with electric current (SPS) for consolidating a sample comprising a substrate and a stack of printed layers in the center of said substrate, the SPS sintering device comprising two movable pistons for applying a uniaxial force along a vertical axis A on said mold to consolidate the stack of layers. The mold comprises: - a first part having a first flat internal face and a first external face; - a second part having a second flat internal face and a second external face; - the two flat internal faces being intended to be placed in contact with the two opposite faces of the flat sample; - each outer face comprising an inclined upper surface, an inclined lower surface, and a straight surface extending between the inclined upper surface and the inclined lower surface, said inclined surfaces being flared towards the straight surface; - an upper assembly element and a lower assembly element each comprising a through hole configured to receive respectively an upper portion and a lower portion of the two parts to form an assembly extending between an upper end and a lower end; - the upper end and the lower end being intended to be brought into contact respectively with a free end of the upper piston and a free end of the lower piston so that the sample held between the two flat internal faces extends in a plane containing the vertical axis A and when the two movable pistons move towards each other to apply a vertical uniaxial force Fv along the axis A on said upper and lower ends, a part of the uniaxial force Fv applied to the upper and lower inclined surfaces of the two parts is decomposed into a tangential component Ft to the inclined surfaces and a normal component FN to the inclined surfaces, the normal component being capable of generating a horizontal compressive force FH applied to the straight surfaces to consolidate the stack of layers.

[0034] The features set out in the following paragraphs may, optionally, be implemented, independently of each other or in combination with each other:

[0035] The upper and lower inclination angles (01, 62) defined respectively between the upper inclined surface and the vertical axis A and the lower inclined surface and the vertical axis A for each of the two parts can be determined so that the horizontal compressive force FH generated by the normal component is less than the minimum working force of the sintering device.

[0036] The upper and lower inclination angles (01, ©2) defined respectively between the upper inclined surface and the vertical axis and the lower inclined surface and the vertical axis for each of the two parts can be between 5 and 10 °.

[0037] The length H3 of the straight surface can correspond to the length of the stack of sample layers printed in the center of the substrate.

[0038] The two parts may be symmetrical about the vertical axis A and when assembled together form a truncated upper portion, a truncated lower portion and a cylindrical central portion extending between the two truncated portions, the truncated portions being flared towards the central portion.

[0039] Both parts and assembly elements can be made of graphite.

[0040] The mold may further comprise at least one temperature sensor, a housing being provided in one of the parts to receive the temperature sensor.

[0041] According to another aspect, there is provided a hot sintering device with SPS electric current configured to consolidate a sample comprising a substrate and a stack of printed layers at the center of said substrate, the device comprising: - a mold as described above; - an upper piston and a lower piston aligned along a vertical main axis A, the two pistons being movable towards each other to respectively apply a vertical uniaxial force along the vertical axis A on an upper end and a lower end of the mold; - an upper electrode and a lower electrode connected respectively to the upper piston and to the lower piston; - at least one upper spacer interposed between the upper electrode and the upper piston and at least one lower spacer interposed between the lower electrode and the lower piston; - a pulsed current generator connected to the upper and lower electrodes; - a vacuum chamber surrounding the mold, electrodes and pistons.

[0042] According to another aspect, there is provided a hot sintering method with pulsed electric current implementing the sintering device as described above for co-sintering a planar sample comprising a substrate and a stack of printed layers at the center of said substrate, the method comprising the following steps: - provide a mold as described above; - place the flat sample between the two internal flat faces of the two parts so that the stack of layers is opposite the straight surfaces of the two parts; - inserting an upper portion and a lower portion of the two parts respectively into the through hole of the upper assembly element and the through hole of the lower assembly element to form the assembly extending between an upper end and a lower end; - placing the mold in the vacuum chamber between a free end of the upper piston and a free end of the lower piston so that the flat sample held between the two flat internal faces of the mold extends in a plane containing the vertical axis A; - applying a vertical uniaxial force along the vertical axis A by moving the two movable pistons towards each other on the upper and lower ends of the mold, a part of the uniaxial force Fv applied on the upper and lower inclined surfaces of the two parts is decomposed into a tangential component Ft to the inclined surfaces and a normal component FN to the inclined surfaces, the normal component being capable of generating a horizontal compressive force FH applied to the straight surfaces to consolidate the stack of layers; - simultaneously applying at least one pulse of electric current to the upper and lower electrodes to cause a rise in temperature in the mold to hot consolidate the stack of layers on the substrate. Brief description of the drawings

[0043] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analyzing the attached drawings, in which: Fig. 1

[0044] [Fig. 1] Figure 1 shows a schematic vertical sectional view of an example of a conventional SPS device comprising a conventional die and pistons. Fig. 2

[0045] [Fig. 2] Figure 2 shows a schematic perspective view of a sample to be consolidated extending in a plane (P) and comprising a substrate and a stack of printed layers in the center of the substrate. Fig. 3

[0046] [Fig. 3] Figure 3 shows a schematic view of a mold according to one embodiment in the presence of the uniaxial vertical force applied by the upper and lower pistons at the ends of the mold and the horizontal compressive force generated at the central area of ​​the mold and an enlarged view showing the decomposition of the uniaxial vertical force into a normal component and a tangential component on the inclined surfaces of the mold. Fig. 4

[0047] [Fig. 4] Figure 4 shows a schematic view showing only the two internal parts of Figure 3. Fig. 5

[0048] [Fig. 5] Figure 5 shows a schematic view of the lower assembly element of the mold of Figure 3. Fig. 6

[0049] [Fig. 6] Figure 6 shows a schematic vertical sectional view of an exemplary SPS device comprising a mold according to one embodiment. Description of the embodiments

[0050] First of all, note that the figures are not to scale.

[0051] In the description, the upper end and the lower end are referred to in relation to the figures, in particular to distinguish them. However, the upper and lower ends may be reversed.

[0052] For the purposes of this disclosure, the term "internal" refers to a portion of the mold close to the vertical central axis A while the term external refers to a portion of the mold further from the vertical central axis A.

[0053] Figures 3, 4, 5 schematically represent a sintering mold 120 for use in a hot sintering device with electric current (SPS) for co-sintering a sample as illustrated in Figure 2.

[0054] The sample 20 has two opposite planar faces which extend parallel to a horizontal main plane P. It comprises a substrate 21 and a stack of layers 22 printed substantially in the center of the substrate.

[0055] For example, the sample can be a piezoelectric micromechanical system whose function is to harvest vibrational mechanical energy and transform it into electrical energy. The stack of layers comprises, for example, an active piezoelectric material sandwiched between two gold electrodes. This stack is printed on a flexible metal substrate. To improve the performance of the system, it is important to optimize the densification of the layers without altering the printed pattern and the integrity of the interfaces.

[0056] It is known to consolidate such a system using a classic cold compression technique, or at low temperature, below 100 °C, and with sintering cycles in classic resistive furnaces. This technique therefore involves two distinct stages, compression and sintering, with long cycles, which generally last several hours and at a high temperature.

[0057] The SPS (Spark Plasma Sintering) technique simplifies the layer consolidation process by simultaneously applying pressure to the sample and an electric current pulse to cause a rapid temperature rise. Thus, the SPS technique allows for very short sintering cycles because compression and sintering are performed in a single step.

[0058] As illustrated in Figure 2, the samples used are flat. They comprise a substrate 21 and a stack of printed layers 22 located in the center of the substrate. The substrate has a width Li between 2 and 6 mm, a length L2 between 30 and 40 mm and a thickness between 100 and 250 pm. The printed layers typically have a thickness between 1 and 100 pm. To be able to densify such a flat sample having a length to width ratio close to 10 with the SPS technique, the sintering mold is designed with a specific design to reduce the force applied to the layers while reducing energy consumption.

[0059] Advantageously, this mold 120 can be used in a standard SPS device as illustrated in Figure 1 which generally comprises two opposing upper and lower electrodes, connected to two upper and lower pistons, and possibly spacers between the electrodes and the pistons. The electrodes, the pistons, the spacers and the mold can be placed in a vacuum or controlled atmosphere chamber. The upper piston and the lower piston are aligned along a vertical central axis A. The two pistons are movable in translation towards each other along this axis to apply a uniaxial force on the mold.

[0060] During operation of the device, an electric current pulse is applied to the electrodes, transmitted to the spacers and pistons and passes through the mold. Simultaneously, a uniaxial vertical force, oriented along the A axis, is exerted on the mold by moving the two pistons. The passage of current through the pistons, the mold and the sample contained in the mold, allows the assembly to be heated by the Joule effect, so as to cause a rapid rise in temperature inside the mold to thus carry out the sintering and compression cycle in a single step. As indicated above, the device allows very high temperatures to be raised, up to 2500°C, very quickly with a temperature rise rate of up to 500°C / minute.

[0061] The mold, pistons and spacers are made of graphite and provide Joule heating to the sample placed in the mold.

[0062] According to one embodiment, the mold comprises a first part 130 and a second part 140. The first part has a first flat internal face 134 and a first external face 135. The second part 140 has a second flat internal face 144 and a second external face 145. The first and second flat internal faces 134 and 144 are intended to be placed in contact with the two opposite flat faces 23, 24 of the sample 20.

[0063] The first outer face 135 has an upper inclined surface 132, a lower inclined surface 133, and a straight surface 131 extending between the upper inclined surface and the lower inclined surface. The second outer face 145 has an upper inclined surface 142, a lower inclined surface 143, and a straight surface 141 extending between the upper inclined surface and the lower inclined surface. The inclined surfaces taper toward the straight surface.

[0064] The parts 130, 140 are assembled together by arranging the two internal flat faces 134, 144 opposite each other, each of the internal flat faces 134, 144 defining a bearing surface for one of the two flat faces of the sample.

[0065] The mold further comprises an upper assembly element 150 and a lower assembly element 160 which are configured to hold the two parts against each other, with the two internal planar faces facing each other as illustrated in FIG. 3.

[0066] Each internal planar face defines a bearing surface for a planar face of the sample. In other words, when using the mold to consolidate the sample, the latter is held between the two internal planar faces 134, 144.

[0067] The upper 150 and lower 160 assembly elements each comprise a through hole 152, 162 configured to respectively receive an upper portion and a lower portion of the two parts to form an assembly.

[0068] Figure 5 shows a sectional view of the lower assembly element 160. It comprises an upper horizontal surface 164, a lower horizontal surface 165, a through hole 162 which extends between the upper surface and the lower surface. The lower through hole 162 defines a frustoconical inner surface which is complementary to the inclined lower outer surface of the first part and to the inclined lower outer surface of the second part. The lower end of the two parts and the lower horizontal surface of the assembly element thus form a planar lower bearing surface 122.

[0069] The upper assembly element 150 is identical to the lower assembly element. The upper through hole 152 defines a frustoconical inner surface which is complementary to the inclined upper outer surface of the first part and to the inclined upper surface of the second part. The upper end of the two parts and the upper horizontal surface of the upper assembly element also form a planar upper bearing surface 121.

[0070] As shown in Figure 3, the formed assembly therefore extends between the upper end 121 forming the upper flat bearing surface intended to be brought into contact with a free end of the lower piston and the lower end 122 forming the lower flat bearing surface intended to be brought into contact with a free end of the lower piston. In this configuration of the arrangement of the mold 120, the sample 20 held between the two flat internal faces 134, 144 therefore extends in a plane which contains the vertical axis A along which the vertical uniaxial force Fv is applied unlike the standard mold where the vertical axis A is perpendicular to the plane of the sample to be densified.

[0071] Due to the presence of the inclined surfaces 132, 142, 133, 143 on an upper portion of the mold and on a lower portion of the mold, when the two movable pistons move towards each other to apply a vertical uniaxial force Fv along the axis A on the upper and lower ends, a part of the uniaxial force Fv is applied to the upper and lower inclined surfaces 132, 142, 133, 143 of the two parts. It is decomposed into a tangential component Ft to the inclined surfaces 132, 142, 133, 143 and a normal component FN to the inclined surfaces 132, 142, 133, 143. This normal component makes it possible to generate a horizontal compressive force FH applied to the two straight external faces 131, 141 of the mold, thus making it possible to consolidate the sample. More precisely, this horizontal compressive force helps consolidate the stack of printed layers in the center of the substrate.

[0072] In Figure 3, an enlarged view of the upper portion of the two parts of the mold illustrates the decomposition of the uniaxial force applied to the upper inclined surface 132 of the first part of the mold into a tangential component Ft and a normal component Fn and the decomposition of the uniaxial force applied to the upper inclined surface 142 of the second part of the mold into a tangential component Ft and a normal component Fn.

[0073] The normal components make it possible to generate a horizontal force on the straight external surfaces of the mold, thus ensuring compression on the layers which are located opposite the straight external surfaces. Preferably and as illustrated in FIG. 3, the length of the straight surface 131, 141 corresponds substantially to the length of the stack of printed layers in the center of the substrate.

[0074] In the example of Figure 3, the length of the substrate corresponds to the length of the mold, that is to say the sum of the three lengths Hi, H2 and H3 indicated in Figure 4. In another example, the length of the substrate can be less than that of the mold.

[0075] Thus, thanks to the specific design of the mold of the present invention, it is possible not to directly apply the vertical force generated by the pistons on the sample but to use a normal component of this force to apply a horizontal compression on the sample. In particular, the minimum working force is no longer applied directly on the sample which can lead to partial or total destruction of the printed layers on the substrate.

[0076] Advantageously, the mold of the present invention introduces a new parameter which is the inclination angle defined between the inclined surface and the vertical axis which makes it possible to adjust the desired horizontal compression force.

[0077] Advantageously, it is possible to determine the upper and lower inclination angles (0i, 82) which are defined respectively between the upper inclined surface and the vertical axis A and the lower inclined surface and the vertical axis A for each of the two parts so that the horizontal compression force FH generated by the normal component is less than the minimum working force of the sintering device.

[0078] The upper and lower inclination angles (0i, 82) defined respectively between the upper inclined surface and the vertical axis and the lower inclined surface and the vertical axis for each of the two parts can be between 5 and 10°. The greater this inclination angle, the greater the horizontal force generated will be.

[0079] Preferably, the two parts are symmetrical with respect to the vertical axis A and form when assembled together a frustoconical upper portion, a frustoconical lower portion and a cylindrical central portion extending between the two frustoconical portions, the frustoconical portions being flared towards the central portion.

[0080] Generally, both parts and the upper and lower assembly elements are made of graphite for the conduction of electric currents during operation of the SPS device.

[0081] According to a variant, the external surfaces of the two parts and the external surfaces of the assembly elements can be covered with a protective layer, in particular a protective layer against oxidation during operation of the SPS device.

[0082] Alternatively, the flat internal surfaces can also be covered with a layer of material to facilitate demolding.

[0083] During operation of the SPS sintering device, the mold acts as a heating source, thanks to the current lines injected by the electrodes and transmitted to the pistons. In order to be able to monitor the temperature rise within the mold and the sample, the mold also has one or more temperature sensors.

[0084] In the example of Figure 3, a housing can be provided in one of the two parts to receive the temperature sensor. Preferably, the housing is provided in an area opposite the layers of the sample that require consolidation, allowing more precision by measuring the temperature near the consolidation area.

[0085] According to another variant, the mold may have several housings allowing several temperature sensors to be received, making it possible to monitor the evolution of the temperature field at several measurement points within the mold.

[0086] This new specific mold for consolidating a flat sample can be used in an SPS sintering device as illustrated in Figure 6.

[0087] Advantageously, compared to a conventional SPS device, with the exception of the mold, the constituent elements of the SPS device are little modified, while the technical advantages obtained are very significant, making it possible to consolidate layers with a thickness of a few tens of microns without destroying the pattern while reducing the volume of material to be heated, thus making it possible to limit energy consumption.

[0088] This SPS device 100 comprises a mold 120 as illustrated in FIG. 3 intended to receive the sample to be densified, a movable upper piston 103, a movable lower piston 104, an upper electrode 107, a lower electrode 108, a current pulse train generator 106, an upper spacer 109 interposed between the upper electrode and the upper piston, a lower spacer 110 interposed between the lower electrode and the lower piston.

[0089] The electrodes, pistons, spacers and mold can be placed in a chamber 112 under vacuum or controlled atmosphere.

[0090] The upper piston 103 and the lower piston 104 are aligned along a vertical main axis A. The two pistons are movable in translation towards each other along this axis to apply a uniaxial force on the mold 120. The two pistons, for example several tens of mm high, have the same diameter as the two ends 121, 122 of the mold 120 which respectively form the upper bearing surface and the lower bearing surface for the mold.

[0091] The device includes a control system for measuring and controlling, in particular, the temperature and pressure inside the chamber, as well as the vertical displacement. Thus, the sintering process can be carried out in temperature control mode using one or more temperature sensors devices within the mold, close to the sample and a PID controller for adjusting the electric current.

[0092] Advantageously, the SPS device may also include temperature sensors placed on the pistons and spacers.

[0093] Each piston 103, 104 has a bearing surface placed at its free end intended to come into contact with the upper end 121 and the lower end 122 of the mold. In this configuration, the sample which is held between the internal flat faces of the mold extends in a plane which contains the vertical axis A along which the uniaxial force is applied.

[0094] The upper electrode 107 and the lower electrode 108 are connected respectively to the upper and lower pistons, and to the pulsed current generator.

[0095] The entire tooling is connected in series, from the upper electrode to the lower one located at the ends of the equipment.

[0096] The mold, pistons and spacers are made of graphite and provide Joule heating to the sample placed in the mold.

[0097] The SPS device using a mold according to the invention makes it possible to use an SPS sintering process to consolidate a stack of low-thickness layers printed in the center of a substrate.

[0098] The sample is pre-prepared with layers of several tens of micrometers printed in the center of the substrate. The layers are typically formed by a piezoelectric layer sandwiched between two gold layers. The substrate is, for example, metallic and flexible. The substrate can be between 30 and 40 mm long and between 2 and 6 mm wide.

[0099] The sample is first placed between the two internal flat faces of the two parts 130, 140 of the mold with the layers facing the straight external surfaces of the two parts.

[0100] To hold the assembly, the upper portion and the lower portion of the two parts are inserted respectively into the through hole of the upper assembly element 150 and the through hole of the lower assembly element 160 to form an assembly which extends between an upper end 121 and a lower end 122.

[0101] Then, the mold is placed in the vacuum chamber with the upper end 121 of the mold in contact with the free end of the upper piston 103 and the lower end 122 of the mold in contact with the free end of the lower piston 104. In this configuration, the sample 20 held between the two flat internal faces 134, 144 of the mold extends in a plane which contains the vertical axis A.

[0102] The mold and sample assembly is flash sintered to consolidate the printed layers in the center of the substrate, in a single step.

[0103] For example, the SPS device used can be programmed to have a temperature rise rate of up to 500°C / min. The uniaxial compressive force exerted by the pistons on the mold can be 50 kN with a minimum force of 3 kN. The sintering temperature is between 25°C and 2500°C. The sintering time can be between 3 minutes and a few hours.

[0104] An electric current pulse is applied to the electrodes 103, 104, transmitted to the spacers 109, 110 and to the pistons 103, 104 and passes through the mold 120. Simultaneously, a uniaxial vertical force, oriented along the axis A, is exerted on the mold by moving the two movable pistons towards each other on the upper and lower ends of the mold.

[0105] A part of the uniaxial force Fv applied to the upper and lower inclined surfaces of the two parts 130, 140 is decomposed into a tangential component Ft to the inclined surfaces and a normal component F n to the inclined surfaces (see Figure 3). The normal component generates a horizontal compressive force FH which is reduced compared to the vertical force and applied to the straight external faces to consolidate the stack of layers.

[0106] The passage of current through the pistons, the mold and the sample contained in the mold allows the assembly to be heated by the Joule effect, so as to cause a rapid rise in temperature inside the mold.

[0107] Under the effect of horizontal pressure and heat, the layers are consolidated together in a single step. Industrial application

[0108] The mold proposed in the present disclosure is particularly suitable for consolidating a multilayer structure formed for example from micrometric piezoelectric material and printed on a large flexible metal substrate.

[0109] Advantageously, the specific geometric shape of the mold allows to reduce the force applied to the multi-layer structure, thus allowing to preserve the functional pattern of the structure, while ensuring the densification of this multi-layer structure.

[0110] Another advantage resulting from the elongated shape of the mold in the direction of the current lines injected by the electrodes allows to reduce the volume of material to be heated, and thus to reduce energy consumption.

[0111] Advantageously, the new mold does not require any changes to the design of the conventional SPS device. Thus, it is possible to take advantage of the efficiency of the SPS device technique in terms of sintering cycle speed to manufacture microelectromechanical systems. The single-step sintering of the piezoelectric structure allows to obtain a system that has both more uniform mechanical strength and good piezoelectric properties.

[0112] This disclosure is not limited to the embodiments described above, but it encompasses all the variants that a person skilled in the art may envisage within the framework of the protection sought.

Claims

Claims

1. A mold (120) for use in a hot sintering device with pulsed electric current (SPS) for consolidating a sample (20) comprising a substrate and a stack of layers (22) printed in the center of said substrate, the SPS sintering device comprising two movable pistons for applying a uniaxial force along a vertical axis A on said mold to consolidate the stack of layers, the mold (120) comprising: - a first part (130) having a first flat internal face (134) and a first external face (135); - a second part (140) having a second flat internal face (144) and a second external face (145); - the two flat internal faces being intended to be placed in contact with the two opposite faces of the flat sample (20); - each external face (135, 145) comprising an inclined upper surface (132, 142), an inclined lower surface (133, 143), and a straight surface (131, 141) extending between the inclined upper surface and the inclined lower surface, said inclined surfaces being flared towards the straight surface; - an upper assembly element (150) and a lower assembly element (160) each comprising a through hole (152, 162) configured to respectively receive an upper portion and a lower portion of the two parts to form an assembly extending between an upper end (121) and a lower end (122); - the upper end (121) and the lower end (122) being intended to be brought into contact respectively with a free end of the upper piston and a free end of the lower piston so that the sample (20) held between the two flat internal faces (134, 144) extends in a plane containing the vertical axis A and when the two movable pistons move towards each other to apply a vertical uniaxial force Fv along the axis A on said upper and lower ends, a part of the uniaxial force Fv applied to the upper and lower inclined surfaces (132, 142, 133, 143) of the two parts is decomposed into a tangential component Ft to the inclined surfaces (132, 142, 133, 143) and a normal component FN to the inclined surfaces (132, 142, 133, 143), the normal component being capable to generate a horizontal compressive force FH applied to the straight surfaces (131, 141) to consolidate the stack of layers.

2. A mold according to claim 1, wherein the through holes define a frustoconical inner surface complementary to the upper and lower inclined surfaces (132, 142, 133, 143), the upper and lower portions of the two parts inserted into the through holes corresponding to the portions having the inclined outer surfaces.

3. A mold according to claim 1 or 2, wherein the upper and lower inclination angles (0i, 62) defined respectively between the upper inclined surface and the vertical axis A and the lower inclined surface and the vertical axis A for each of the two parts are determined so that the horizontal compression force FH generated by the normal component is less than the minimum working force of the sintering device.

4. Mold according to one of claims 1 to 3, in which the upper and lower inclination angles (0i, 62) defined respectively between the upper inclined surface and the vertical axis and the lower inclined surface and the vertical axis for each of the two parts are between 5 and 10 °.

5. Mold according to one of claims 1 to 4, in which the length H3 of the straight surface (131, 141) corresponds to the length of the stack of layers of the sample printed in the center of the substrate.

6. Mold according to one of claims 1 to 5, in which the two parts are symmetrical with respect to the vertical axis A and form when assembled together a frustoconical upper portion, a frustoconical lower portion and a cylindrical central portion extending between the two frustoconical portions, the frustoconical portions being flared towards the central portion.

7. Mold according to one of claims 1 to 6, in which the two parts and the assembly elements are made of graphite.

8. Mold according to one of claims 1 to 7, further comprising at least one temperature sensor (105), a housing being provided in one of the parts to receive the temperature sensor.

9. Device (100) for hot sintering with pulsed electric current (SPS) configured to consolidate a sample (20) comprising a substrate (21) and a stack of layers (22) printed in the center of said substrate, the device comprising: - a mold (120) according to one of claims 1 to 8; - an upper piston (103) and a lower piston (104) aligned along a vertical main axis A, the two pistons being movable towards each other to respectively apply a vertical uniaxial force along the vertical axis A on an upper end (121) and a lower end (122) of the mold; - an upper electrode (107) and a lower electrode (108) connected respectively to the upper piston (103) and to the lower piston (104); - at least one upper spacer (109) interposed between the upper electrode and the upper piston and at least one lower spacer (110) interposed between the lower electrode and the lower piston; - a pulsed current generator (106) connected to the upper (107) and lower (108) electrodes; - a vacuum chamber (112) surrounding the mold, the electrodes and the pistons.

10. A method of hot sintering with electric current using the sintering device of claim 9 for co-sintering a planar sample (20) comprising a substrate (21) and a stack of layers (22) printed in the center of said substrate, the method comprising the following steps: - providing a mold (120) according to one of claims 1 to 8; - placing the flat sample (20) between the two internal flat faces (134, 144) of the two parts (130, 140) so that the stack of layers (22) is opposite the straight surfaces of the two parts; - inserting an upper portion and a lower portion of the two parts respectively into the through hole (152) of the upper assembly element (150) and the through hole (162) of the lower assembly element (160) to form the assembly extending between an upper end (121) and a lower end (122); - placing the mold in the vacuum chamber (112) between a free end of the upper piston and a free end of the lower piston so that the flat sample (20) held between the two flat internal faces (134, 144) of the mold extends in a plane containing the vertical axis A; - applying a vertical uniaxial force along the vertical axis A by moving the two movable pistons towards each other on the upper and lower ends of the mold, a part of the uniaxial force Fv applied to the upper and lower inclined surfaces (132, 142, 133, 143) of the two parts is decomposed into a tangential component Ft to the inclined surfaces (132, 142, 133, 143) and a normal component FN to the inclined surfaces (132, 142, 133, 143), the normal component being capable of generating a horizontal compressive force FH applied to the straight surfaces to consolidate the stack of layers; - simultaneously applying at least one pulse of electric current to the upper and lower electrodes to cause a rise in temperature in the mold to hot consolidate the stack of layers on the substrate.

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