Mold for pulsed electric current assisted sintering device

Abstract

A mold (120) intended for use in an SPS sintering device to consolidate a sample (20) comprising a substrate and a stack of layers (22) printed at the center of said substrate, the SPS sintering device comprising two movable pistons to apply a uniaxial force along a vertical axis A on said mold to consolidate the stack of layers. The mold (120) is configured so that the sample (20) is held between two flat internal faces (134, 144) and extends in a plane containing the vertical axis A. When the two movable pistons move towards each other to apply a vertical uniaxial force FV along the axis A, a portion of the uniaxial force FV applied to the mold is decomposed into a tangential component Ft and a normal component FN. The normal component is capable of generating a horizontal compressive force FH to consolidate the layer stack. (Abstract figure: Figure 3)

Description

Title of the invention: Mold for a pulsed electric current assisted sintering device Technical field

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

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

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

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

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

[0007] This SPS device comprises a die 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 to apply a uniaxial force about a vertical axis A on the powder to be densified. Each piston has a bearing surface at its free end for contacting 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 respectively to be connected to the pulsed current generator 6.

[0010] The device 1 also includes spacers 9, 10 interposed between the electrode and the piston. In the example shown in [Fig. 1], the device includes two upper spacers interposed between the upper electrode and the end of the upper piston opposite its bearing surface, and 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 by means of an assembly of pistons and spacers.

[0012] The device also includes one or more temperature probes 5, such as thermocouples, which allow the sintering temperature to be monitored and controlled for regulation by control electronics (not shown). As mentioned above, the device allows for very high temperatures, up to 2500°C, to be reached very rapidly, with a heating rate of up to 500°C / minute.

[0013] The matrix, pistons, spacers and electrodes are placed inside a chamber 12 under vacuum or controlled atmosphere.

[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 generally made of graphite and provide Joule heating to the sample loaded in the center of the die. The inner wall of the die is lined 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 [Fig.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 flows through the spacers, pistons, and sample in successive bursts at a defined frequency, enabling a very rapid temperature rise and complete sintering in a few minutes. This technique allows, in particular, precise control of the material's microstructure down to the nanometer scale.

[0018] The current is applied in the form of current pulse trains, 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 [Fig.1], a ceramic piece to be densified, or two pieces, for example ceramic pieces to be assembled to obtain a stack of pieces.

[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 matrix and also the sample, in powder form, when the latter is conductive.

[0021] Thus, the matrix, spacers, and pistons act as a heating source, enabling high heating rates. Unlike conventional sintering, the sample is heated from the outside in by thermal conduction from the matrix.

[0022] The SPS technique therefore offers numerous technological and economic advantages compared to 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, on the order of a few tens of minutes. This rapid sintering makes it possible to limit grain growth and, for some materials, achieve a density close to 100%, thus improving the mechanical properties of the densified materials.

[0023] One of the major constraints of the SPS process stems from the design of the SPS equipment, and in particular from the shape and dimensions of the matrix. Indeed, the geometric shape and its dimensions 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 samples with large diameters. Furthermore, when starting the device, a minimum force must be applied to the sample to ensure safe operation, and this minimum force is applied almost instantaneously. Therefore, the mold must have a minimum size to withstand this minimum force.

[0025] By way of 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 arises from the mold design, the geometric shape and dimensions of which are reserved for the manufacture of small parts, generally with a diameter between 10 and 20 mm and a thickness between 2 and 100 mm, or even 1000 mm, exhibiting a geometric shape of cylinder.

[0027] In the context of the present invention, and as illustrated in [Fig. 2], the sample to be sintered 20 has an elongated, 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 generally has a thickness of between 100 and 250 µm. It also includes a stack of printed layers 22 to be co-sintered located at the center of the substrate. The stack of layers to be co-sintered, on the other hand, has a dimension of between 50 and 600 µm and includes a functional pattern that 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 barely reach the desired working temperature, or even fail to reach it at all. Furthermore, 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] The use of 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 suddenly applying the minimum force to the stack of layers can lead to partial or total destruction of the printed pattern.

[0030] It follows from the above that there is a need for a mold intended for use in a standard SPS device, and an SPS process 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 deteriorating the imprinted pattern of the central stack of layers.

[0031] Another object of this disclosure is to propose a mold with a specific geometric shape that reduces the volume of material to be heated in order to reduce energy consumption compared to a conventional mold, and ensures a homogeneous temperature distribution when used in an SPS device. Summary

[0032] This disclosure improves the situation.

[0033] A mold is proposed for use in a static hot sintering (SPS) device to consolidate a sample comprising a substrate and a stack of printed layers at 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 presenting a first flat inner face and a first outer face; - a second part having a second flat inner face and a second outer face; - the two flat internal faces being intended to be brought into contact with the two opposite faces of the flat sample; - each external 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 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 surface the normal component being able to generate a horizontal compressive force FH applied on the straight surfaces to consolidate the stacking of layers.

[0034] The features described 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 (0i, 02) 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 (0i, 02) defined respectively between the The angle between the upper inclined surface and the vertical axis and between 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 layers of the sample printed at the center of the substrate.

[0038] The two parts can be symmetrical with respect to the vertical axis A and when assembled together form 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.

[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, a hot sintering device with electric current SPS is proposed configured to consolidate a sample comprising a substrate and a stack of layers printed in the center of said substrate, the device comprising: - a mold as described above; - an upper piston and a lower piston aligned along a main vertical axis A, the two pistons being movable towards each other to apply respectively 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 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, a hot sintering process with pulsed electric current is proposed, implementing the sintering device as described above for co-sintering a flat sample comprising a substrate and a stack of layers printed at the center of said substrate, the process 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 stacking of layers is opposite the straight surfaces of the two parts; - insert 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; - Place the mold in the vacuum chamber between one free end of the piston upper and a free end of the lower piston so that the flat sample held between the two flat inner faces of the mold extends in a plane containing the vertical axis A; - apply 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, 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 surface, the normal component being able to generate a horizontal compressive force FH applied on the straight surfaces to consolidate the stacking of layers; - apply at least one electric current pulse simultaneously to the upper and lower electrodes to cause a temperature rise 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 analysis of the accompanying drawings, on which: Fig. 1

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

[0045] [Fig.2] Fig.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 at the center of the substrate. Fig. 3

[0046] [Fig.3] Fig.3 shows a schematic view of a mold according to an 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 compression 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] The [Fig.4] shows a schematic view showing only the two internal parts of the [Fig.3]. Fig. 5

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

[0049] [Fig.6] Fig.6 shows a schematic vertical cross-sectional view of an example of an SPS device comprising a mold according to one embodiment. Description of the implementation methods

[0050] First of all, it should be noted that the figures are not to scale.

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

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

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

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

[0055] By way of example, the sample may be a piezoelectric micromechanical system designed to recover vibratory mechanical energy and convert it into electrical energy. The layer stack comprises, for example, an active piezoelectric material sandwiched between two gold electrodes. This stack is printed onto a flexible metallic substrate. To improve the system's performance, it is important to optimize the layer density without altering the printed pattern and the integrity of the interfaces.

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

[0057] The Spark Plasma Sintering (SPS) technique simplifies the layer consolidation process by simultaneously applying pressure to the sample and an electric current pulse to the sample to induce 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 [Fig. 2], the samples used are flat. They comprise a substrate 21 and a stack of printed layers 22 located at the center of the substrate. The substrate has a width Li of between 2 and 6 mm, a length L2 of between between 30 and 40 mm and a thickness between 100 and 250 µm. The printed layers typically have a thickness between 1 and 100 µm. To densify such a flat sample with a length-to-width ratio close to 10 using the SPS technique, the sintering mold is designed with a specific design to reduce the force applied to the layers while also reducing energy consumption.

[0059] Advantageously, this mold 120 can be used in a standard SPS device as illustrated in [Fig. 1], which generally comprises two opposing upper and lower electrodes connected to two upper and lower pistons, and optionally spacers between the electrodes and the pistons. The electrodes, pistons, spacers, and mold can be placed in a vacuum chamber or under a controlled atmosphere. The upper and lower pistons 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 to 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 axis A, is exerted on the mold by the movement of the two pistons. The passage of current through the pistons, the mold, and the sample contained within the mold heats the entire assembly by Joule heating, causing a rapid temperature rise inside the mold to complete the sintering and compression cycle in a single step. As mentioned above, the device allows for very high temperatures, up to 2500°C, to be reached 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 effect 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 inner face 134 and a first outer face 135. The second part 140 has a second flat inner face 144 and a second outer face 145. The first and second flat inner faces 134 and 144 are intended to be in contact with the two opposite flat faces 23, 24 of the sample 20.

[0063] The first external face 135 comprises 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 external face 145 comprises 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 flare outwards towards 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 support surface for one of the two flat faces of the sample.

[0065] The mold further includes 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 flat faces opposite each other as illustrated in [Fig.3].

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

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

[0068] Figure 5 shows a cross-sectional view of the lower assembly element 160. It comprises an upper horizontal surface 164, a lower horizontal surface 165, and a through hole 162 extending between the upper and lower surfaces. The lower through hole 162 defines a frustoconical inner surface that is complementary to the inclined lower external surface of the first part and the inclined lower external surface of the second part. The lower end of the two parts and the lower horizontal surface of the assembly element thus form a flat 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 that is complementary to the inclined upper outer surface of the first part and 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 flat upper bearing surface 121.

[0070] As shown in [Fig. 3], the resulting assembly extends between the upper end 121, forming the upper flat bearing surface intended to be in contact with a free end of the lower piston, and the lower end 122, forming the lower flat bearing surface intended to be in contact with a free end of the lower piston. In this configuration of the mold 120, the sample 20, held between the two internal flat faces 134, 144, therefore extends in a plane that 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] Thanks to the presence of 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 axis A at the upper and lower extremities, a portion of the uniaxial force Fv is applied to the upper and lower inclined surfaces 132, 142, 133, 143 of both parts. This force is decomposed into a tangential component Ft at the inclined surfaces 132, 142, 133, 143 and a normal component FN at the inclined surfaces 132, 142, 133, 143. This normal component generates a horizontal compressive force FH applied to the two straight outer faces 131, 141 of the mold, thus consolidating the sample. More precisely, this horizontal compressive force consolidates the stack of printed layers at the center of the substrate.

[0072] On [Fig.3], an enlarged view of the upper portion of the two parts of the mold illustrates the decomposition of the uniaxial force applied on 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 on 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 generate a horizontal force on the straight external surfaces of the mold, thus ensuring compression on the layers located opposite these straight external surfaces. Preferably, and as illustrated in [Fig. 3], the length of the straight surface 131, 141 corresponds approximately to the length of the stack of printed layers at the center of the substrate.

[0074] In the example of [Fig.3], the length of the substrate corresponds to the length of the mold, i.e. the sum of the three lengths Hb H2 and H3 indicated on [Fig.4]. In another example, the length of the substrate may be less than that of the mold.

[0075] Thus, thanks to the specific design of the mold of the present invention, it is possible to avoid directly applying the vertical force generated by the pistons to the sample, but rather to use a normal component of this force to apply horizontal compression to the sample. In particular, the minimum working force is no longer applied directly to the sample, which could lead to partial or total destruction of the layers printed on the substrate.

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

[0077] Advantageously, it is possible to determine the upper and lower inclination angles (0i, 02) 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 compressive 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, 02) 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 when assembled together form 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 electrical currents during operation of the SPS device.

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

[0082] According to another variant, the flat internal surfaces can also be covered with a layer of material which makes demolding easier.

[0083] During the 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 monitor the temperature rise within the mold and the sample, the mold also includes one or more temperature sensors.

[0084] As illustrated in [Fig. 3], a housing can be provided in one of the two parts to accommodate the temperature sensor. Preferably, the housing is located in an area adjacent to the sample layers that require consolidation, allowing for greater accuracy by measuring the temperature near the consolidation zone.

[0085] According to another variant, the mold may include 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 [Fig.6].

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

[0088] This SPS 100 device includes 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 interposed 109 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 under a controlled atmosphere.

[0090] The upper piston 103 and the lower piston 104 are aligned along a main vertical 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 and lower bearing surfaces 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 within the mold, near the sample, and a PID controller for adjusting the electrical 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 at its free end intended to contact the upper end 121 and the lower end 122 of the mold. In this configuration, the sample held between the internal flat faces of the mold extends in a plane containing 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 effect heating to the sample placed in the mold.

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

[0098] The sample is previously prepared with layers of several tens of Micrometers are printed in the center of the substrate. The layers are typically formed of 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 opposite the straight external surfaces of the two parts.

[0100] To hold the assembly together, 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] Next, 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 inner faces 134, 144 of the mold extends in a plane which contains the vertical axis A.

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

[0103] By way of example, the SPS device used can be programmed to achieve a temperature rise rate of up to 500°C / min. The uniaxial compression force exerted by the pistons on the mold can be 50 kN, with a minimum force of 3 kN. The sintering temperature ranges from 25°C to 2500°C. The sintering time can range from 3 minutes to several 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 portion 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 Fn to the inclined surface (see [Fig. 3]). The normal component generates a horizontal compressive force FH, which is reduced relative to the vertical force and applied to the straight external faces to consolidate the layer stack.

[0106] The passage of current through the pistons, the mold and the sample contained in the mold, allows the whole to be heated by Joule effect, so as to cause a rapid temperature rise 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 this disclosure is particularly suitable for consolidating a multilayer structure formed, for example, of micrometric piezoelectric material printed on a large flexible metallic substrate.

[0109] Advantageously, the specific geometric shape of the mold makes it possible to reduce the force applied to the multilayer structure, thus preserving the functional pattern of the structure, while ensuring the densification of this multilayer structure.

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

[0111] Advantageously, the new mold does not require any modifications to the design of the conventional SPS device. Thus, it is possible to leverage the efficiency of the SPS device technique in terms of sintering cycle speed to fabricate microelectromechanical systems. The single-step sintering of the piezoelectric structure results in a system that exhibits both more homogeneous mechanical strength and good piezoelectric properties.

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

Claims

Demands

1. Mold (120) for use in a pulsed electric current hot sintering (PSS) device for consolidating a sample (20) comprising a substrate and a stack of layers (22) printed at the center of said substrate, the SPS sintering device comprising two movable pistons for applying a uniaxial force about a vertical axis A on said mold for consolidating the stack of layers, the mold (120) comprising: - a first part (130) having a first flat inner face (134) and a first outer face (135); - a second part (140) having a second flat inner face (144) and a second outer face (145); - the two flat internal faces being intended to be brought into 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 receive respectively 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 such that the sample (20) held between the two flat inner 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 on 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 force of com-; FH pressure applied to the straight surfaces (131, 141) to consolidate the stacking of layers.

2. Mold according to claim 1, wherein the through holes define an internal frustoconical 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 external inclined surfaces.

3. Mold according to claim 1 or 2, wherein the upper and lower inclination angles (Oi, 02) 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 such that the horizontal compressive force FH generated by the normal component is less than the minimum working force of the sintering device.

4. Mold according to any one of claims 1 to 3, wherein the upper and lower inclination angles (Oi, 02) 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 any one of claims 1 to 4, wherein the length H3 of the straight surface (131, 141) corresponds to the length of the stack of layers of the sample printed at the center of the substrate.

6. Mold according to any one of claims 1 to 5, wherein the two parts are symmetrical with respect to the vertical axis A and when assembled together form 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 any one of claims 1 to 6, wherein both parts and assembly elements are made of graphite.

8. Mold according to any 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. A pulsed electric current hot sintering (PSS) device (100) configured to consolidate a sample (20) comprising a substrate (21) and a stack of layers (22) printed at the center of said substrate, the device comprising: - a mold (120) according to any one of claims 1 to 8; - an upper piston (103) and a lower piston (104) aligned along a main vertical axis A, the two pistons being movable towards each other to apply respectively 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 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, electrodes and pistons.

10. A hot electric current sintering process implementing 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 process comprising the following steps: - to supply a mold (120) according to any one of claims 1 to 8; - place the planar sample (20) between the two internal planar 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; - insert 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); - place 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 inner faces (134, 144) of the mold extends in a plane containing the vertical axis A; - apply 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, part of the uniaxial force Fv applied on 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 able to generate a horizontal compressive force Fh applied to the straight surfaces to consolidate the stacking of layers; - apply at least one electric current pulse simultaneously to the upper and lower electrodes to cause a temperature rise in the mold to hot consolidate the stack of layers on the substrate.

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