JP7602302B2 - Spark plasma sintering method - Google Patents

Description

The present invention relates to a spark plasma sintering technique in which powder material is sintered by passing an electric current through it while being pressurized. More specifically, the present invention relates to a spark plasma sintering apparatus and a spark plasma sintering method that make it possible to effectively remove gases present between the particles of the powder material and impurities attached to the particles.

Spark plasma sintering (also called electric current sintering or pulse current pressure sintering) is used as a sintering method for metals, ceramics, etc. Spark plasma sintering is a method of sintering a material by applying pressure to a powdered material while applying pulse current heating. Compared to hot press sintering and hot isostatic pressure sintering, which have been used as manufacturing methods for sintered bodies, spark plasma sintering has the following advantages: it can sinter at low temperatures in a short time, it has good moldability and can sinter materials that could not be molded by conventional sintering methods, it can produce dense sintered bodies with controlled grain growth, and it consumes less power than hot press sintering. These advantages have attracted attention in recent years.

The general configuration of a conventional discharge plasma sintering apparatus is as shown in the schematic diagram of FIG. 9. The discharge plasma sintering apparatus 5 includes a cylindrical die 52 having an internal space in which the powder material M is filled, and two punches 53, 54 inserted into the internal space from the upper and lower ends of the die 52. The die 52 and the two punches 53, 54 are usually formed using graphite. The die 52 and the two punches 53, 54 are arranged inside the vacuum chamber 50 so as to be sandwiched between electrodes 57, 58 for applying a pulse current to the powder material M via spacers 55, 56. The lower punch 54 is movable in the axial direction of the cylindrical die 52 by a press mechanism 70, thereby pressurizing the powder material M filled in the internal space of the die 52. When performing spark plasma sintering, powder material M is placed in the internal space of die 52, punches 53 and 54 are inserted into the internal space until they come into contact with the powder material, and the combination of die 52 and punches 53 and 54 is placed in vacuum chamber 50. Next, open/close valve 62 is opened, and air is evacuated from inside vacuum chamber 50 by vacuum pump 61. Press mechanism 70 is operated to apply pressure to powder material M with punches 53 and 54, while applying pulsed electricity to powder material M with electrodes 57 and 58. This produces a sintered body from powder material M.

In order to stabilize the physical properties of the sintered body, it is desirable to use powder material filled into the die that is as pure as possible. However, there are spaces between the particles of the filled powder material, and these spaces contain gases such as air and foreign matter. Furthermore, it is inevitable that gases such as oxygen and nitrogen contained in the air, moisture, oxide films, and foreign matter such as sebum that has adhered during handling will adhere to the surfaces of each particle of the powder material.

JP 2006-131921 A Japanese Patent Application Publication No. 9-324228 JP 2003-49207 A

Patent Document 1 proposes an apparatus that has a glove box-type pre-processing chamber connected to the sintering process chamber and controlled to a specific atmosphere in order to remove gases and foreign matter present in the spaces between particles and gases and foreign matter attached to the particle surfaces (hereinafter referred to as "impurities"). In this apparatus, the raw material powder is weighed in the pre-processing chamber under an inert gas atmosphere, filled into the sintering mold (die), punches are inserted into the sintering mold, and the chamber pressure is reduced, and this is transferred to the sintering process chamber without exposure to the outside air, which is said to enable the production of sintered bodies that are free from contamination during the preparation of the raw material powder. However, this technology requires large-scale and complicated equipment and operations, and the constant use of inert gas poses problems with the impact on the environment.

Technologies for removing impurities between particles and on the particle surfaces are also disclosed in Patent Document 2 and Patent Document 3. Patent Document 2 discloses that metal powder is filled between a graphite die and upper and lower punches that also serve as electrodes, and this is placed on a sintering stage inside a container, and the container is depressurized to remove gases and the like adsorbed to the metal powder. Patent Document 3 also discloses that a container containing aluminum alloy powder is heated while the air inside the container is reduced to a vacuum atmosphere, thereby removing material degradants such as water and hydroxides from the aluminum alloy powder.

However, as disclosed in Patent Document 2, sufficient impurity removal cannot be achieved by simply removing gases and the like adsorbed to the metal powder by reducing the pressure. In addition, the technology disclosed in Patent Document 3 requires reducing the pressure while heating a metal container containing raw material powder, compressing the raw material powder inside to a predetermined bulk density to obtain a sintered body, and then removing the sintered body from the container. Therefore, there are many steps and the process is complicated.

In a spark plasma sintering apparatus, the die and punch are usually fitted with high precision to prevent any gaps from forming for the powder material to escape, and the fitting gap between the inner wall of the die and the punch is extremely small. In addition, when the punch comes into contact with the powder material filled in the die, it is thought that the powder material may get into the boundary between the die and the punch. Therefore, simply creating a vacuum with the punch inserted in the die does not easily remove impurities between the particles of the powder material or on the particle surfaces. If the powder material is sintered without removing the impurities, the impurities may be absorbed in the sintered body, reducing the purity. In addition, voids that remain under high pressure inside the sintered body may become the starting point of cracks, reducing the mechanical strength of the sintered body.

The objective of the present invention is to provide a spark plasma sintering apparatus and a spark plasma sintering method that can remove impurities between particles of powder material and impurities attached to the particle surfaces more effectively than conventional methods.

The inventors of the present invention discovered that impurities between particles of powder material and impurities attached to the particle surfaces can be more effectively removed by evacuating the chamber with the space filled with powder material open and removing impurities by heating with the space filled with powder material open, or by removing impurities by heating multiple times with the space filled with powder material open, and thus completed the present invention.

In a first aspect, the present invention provides a spark plasma sintering apparatus that sinters a powder material filled in an internal space of a cylindrical die by applying pressure to the powder material with a punch in a vacuum atmosphere while applying current to the powder material. The spark plasma sintering apparatus includes a first punch and a second punch that are inserted into the internal space of the die, an electrode for applying current to the first and second punches, and a moving mechanism that moves at least one of the first punch and the die so that the first punch can be inserted into the internal space and removed from the internal space. A punch holding part that detachably holds the first punch is disposed between the first punch and the electrode that applies current to the first punch. The first punch has a corresponding held part that can be held by the punch holding part. In one embodiment, a spacer can be disposed between the first punch and the electrode that applies current to the first punch, and the punch holding part can be provided on the spacer.

In one embodiment, the held portion of the first punch and the punch holding portion are preferably configured to allow the first punch to move. By allowing the first punch to move, the first punch can be automatically aligned with the internal space when the first punch is inserted into the internal space in a vacuum atmosphere by operation of the moving mechanism. To achieve this, in one embodiment, it is preferable that the held portion of the first punch has a flange, and the punch holding portion has a hook that engages with the flange.

In one embodiment, the die preferably has a guide portion for aligning the first punch with the internal space. The guide portion is preferably a tapered portion formed toward the back of the internal space at the entrance portion of the die on the side where the first punch is inserted.

In a second aspect, the present invention provides a spark plasma sintering method for sintering a powder material filled in the internal space of a cylindrical die by applying current to the powder material while applying pressure to the powder material with a punch in a vacuum atmosphere. The spark plasma sintering method includes a vacuum process for creating a vacuum atmosphere around the die filled with the powder material, a heating process for heating the powder material, a holding process for holding the heated powder material without the punch being inserted into the internal space, and a sintering process for inserting a punch into the internal space and applying current to the powder material while applying pressure to the powder material with the punch. In the vacuum process, it is more preferable to create a vacuum atmosphere around the die filled with the powder material without the punch being inserted into the internal space of the die.

In one embodiment, the heating step preferably includes inserting a punch into the internal space, heating the powder material by passing an electrical current through the punch, and then removing the punch from the internal space.

According to the present invention, since the punch can be inserted into the die and removed from the die in a vacuum atmosphere without the worker touching the die or punch, it is possible to perform non-thermal impurity removal and heated impurity removal consecutively in a series of processes, or to perform the heated impurity removal process multiple times. Therefore, the device and method according to the present invention can effectively remove impurities, reducing variation in purity, and can produce sintered bodies of stable quality that are densified by evacuating the powder space.

1 is a diagram showing a configuration of a main part of a spark plasma sintering apparatus according to an embodiment of the present invention. 1A and 1B are diagrams showing an upper punch used in a spark plasma sintering apparatus, in which FIG. 1A is a vertical cross-sectional view and FIG. 1A and 1B are diagrams showing an upper spacer used in a spark plasma sintering apparatus, in which FIG. 1A is a vertical cross-sectional view and FIG. FIG. 2 is a vertical cross-sectional view showing a die used in a spark plasma sintering apparatus. FIG. 13 illustrates how the upper punch is automatically aligned when it is inserted into the inner space of the die. FIG. 1 is a flow diagram showing steps of a spark plasma sintering method according to an embodiment of the present invention. 13 is an EDS element mapping image of a sintered body sample from which impurities were removed by heating. 13 is an EDS element mapping image of a sintered body sample that was not subjected to thermal impurity removal. FIG. 1 is a diagram showing a configuration of a main part of a conventional spark plasma sintering apparatus.

The following describes in detail an embodiment of the present invention with reference to the drawings. The present invention is not limited to the following embodiment, and can be modified as appropriate within the scope of the object of the present invention.

[Configuration of Spark Plasma Sintering Apparatus]
FIG. 1 is a schematic diagram showing the configuration of the main parts of a discharge plasma sintering apparatus 1 (hereinafter, simply referred to as the apparatus 1) according to one embodiment of the present invention. The main parts shown in FIG. 1 are housed inside a housing (not shown) of the apparatus 1. The apparatus 1 includes a cylindrical die 12 having an inner space 11, and an upper punch 13 and a lower punch 14 inserted into the inner space 11 of the die 12. The upper punch 13 is inserted into the inner space 11 from above the die 12, and the lower punch 14 is inserted into the inner space 11 from below the die 12. The shapes of the parts of the upper punch 13 and the lower punch 14 inserted into the inner space 11 have shapes that match the inner space 11. The inner space 11 is filled with a powder material M, and the powder material M becomes a sintered body by passing current while being pressurized by the upper punch 13 and the lower punch 14.

The die 12, upper punch 13, and lower punch 14 are preferably made of a conductive material. Specifically, they are preferably made of graphite, steel material, silicon carbide, tungsten carbide, etc., and more preferably made of graphite. The size and shape of the die 12 and inner space 11, as well as the upper punch 13 and lower punch 14, are not particularly limited and can be designed appropriately according to the size and shape of the desired sintered body and the scale of the equipment. In addition, the die 12, upper punch 13, and lower punch 14 are formed so that the fitting gap between the inner wall of the die 12 and the upper punch 13 and lower punch 14 is extremely small so that no escape gap for the powder material M occurs (high pressure will not be generated if an escape gap exists).

The upper punch 13 is connected to an upper electrode 17 via an upper spacer 15, and the lower punch 14 is connected to a lower electrode 18 via a lower spacer 16. The upper spacer 15 and the lower spacer 16 are also formed of a conductive material similar to that of the die 12, the upper punch 13, and the lower punch 14, and are preferably formed of graphite. The upper electrode 17 and the lower electrode 18 are connected to a power source (not shown) that generates DC pulse power or single-phase full-wave power. The power generated by the power source is applied to the die 12 (or to the powder material M itself if the powder material M is a conductive material) via the upper electrode 17 and the lower electrode 18, the upper spacer 15 and the lower spacer 16, and the upper punch 13 and the lower punch 14.

The die 12, the upper punch 13 and the lower punch 14, the upper spacer 15 and the lower spacer 16, and a part of the upper electrode 17 and the lower electrode 18 are arranged inside the vacuum chamber 10. The inside of the vacuum chamber 10 can be made into a vacuum atmosphere by opening the opening/closing valve 22 and evacuating the air with the vacuum pump 21.

The upper electrode 17 is fixed at a position inside the device 1, and the lower electrode 18 is movable up and down by a press mechanism (moving mechanism) 30 (not shown). When the press mechanism 30 moves the lower electrode 18 upward, the lower punch 14 moves upward inside the internal space 11, and high pressure can be applied to the powder material M filled in the internal space 11. Note that the vacuum chamber 10 is configured so that the lower electrode 18 can be moved up and down while maintaining the inside in a vacuum. In this embodiment, the lower electrode 18 is configured to be moved by the press mechanism 30, but this is not limited thereto, and the upper electrode 17 may be configured to be movable, or both the upper electrode 17 and the lower electrode 18 may be configured to be movable.

(Upper spacer and upper punch)
Fig. 2 is a diagram showing an upper punch 13 used in the device 1, (a) being a vertical cross-sectional view and (b) being a bottom view. Also, Fig. 3 is a diagram showing an upper spacer 15 used in the device 1, (a) being a vertical cross-sectional view and (b) being a bottom view.

The upper punch 13 comprises a cylindrical body 131 and a retained portion for being retained by the upper spacer 15. In this embodiment, the retained portion is a ring-shaped flange 132 provided to protrude from the side surface of the body 131 as shown in FIG. 2. Meanwhile, the upper spacer 15 comprises a cylindrical body 151 and a punch retaining portion for removably retaining the upper punch 13. In this embodiment, the punch retaining portion is hooks 152, 153 provided to protrude downward (i.e., toward the upper punch 13) from the underside of the body 151. The hooks 152, 153 have protruding portions 152a, 153a formed to face each other as shown in FIG. 3.

In this embodiment, as shown in FIG. 1, the upper punch 13 is placed so that the lower surface of the flange 132 contacts the upper surface of the protruding portions 152a, 153a of the upper spacer 15. Therefore, the upper punch 13 can move freely laterally even when engaged with the upper spacer 15. With this structure, the press mechanism 30 moves the lower electrode 18 up and down, so that the upper punch 13 can be inserted into the internal space 11 of the die 12 in a vacuum atmosphere, and the upper punch 13 can be removed from the internal space 11 of the die 12. In addition, when the press mechanism 30 moves the lower electrode 18 upward to insert the upper punch 13 into the internal space 11 of the die 12 in a vacuum atmosphere, even if the central axis of the upper punch 13 does not coincide with the central axis of the internal space 11, the upper punch 13 moves laterally and is automatically aligned with the internal space 11. This point will be explained again with reference to FIG. 5 in the description of the die 12 described later.

In another embodiment, the upper punch 13 may be removably attached to the upper spacer 15 by screwing the male screw into the female screw, so that the upper punch 13 can be removably attached to the upper spacer 15. This structure makes it possible to insert the upper punch 13 into the inner space 11 of the die 12 in a vacuum atmosphere or remove the upper punch 13 from the inner space 11 of the die 12 by the press mechanism 30 moving the lower electrode 18 up and down. In yet another embodiment, the upper punch 13 can be removably attached to the upper spacer 15 using, for example, a pin that penetrates the hooks 152 and 153 of the upper spacer 15 and reaches the upper punch 13. In yet another embodiment, the upper punch 13 can be removably attached to the upper spacer 15 by attaching a wire to an appropriate position, such as the inner space of the upper spacer 15, and hanging the upper punch 13 from the wire.

In yet another embodiment, when the upper spacer 15 is not used, i.e., when the upper punch 13 is in direct contact with the upper electrode 17, a punch holding portion (hooks 152, 153 in the case of FIG. 1) may be provided on the lower surface of the upper electrode 17. For example, a hook portion as shown in FIG. 3 may be provided so as to protrude downward from the lower surface of the upper electrode 17.

(Thailand)
FIG. 4 is a vertical cross-sectional view showing the die 12 used in the device 1. The die 12 preferably has a guide portion at the entrance portion 111 where the upper punch 13 is inserted to assist in aligning the upper punch 13 with the inner space 11. In this embodiment, as shown in FIG. 1 and FIG. 4, the die 12 preferably has a shape in which the corner facing the inner space 11 is cut out over the entire circumferential direction at the entrance portion 111 on the side where the upper punch 13 is inserted. That is, the guide portion of the die 12 in this embodiment is a tapered portion 122 provided at the corner facing the inner space 11 so that the diameter of the inner space 11 gradually narrows from the entrance portion 111 toward the back of the inner space 11. By providing the tapered portion 122 at the entrance portion 111 of the die 12 in this way, it becomes easier to align the upper punch 13 with the inner space 11 when inserting the upper punch 13 in a facing atmosphere. The angle (angle θ in FIG. 4) between two points when the tapered portion 122 is lowered into the internal space 11 along the tapered portion 122 at opposing positions in the radial direction of the die 12 is preferably, but not limited to, 45° to 80°. If this angle is too small, the tolerance for misalignment between the die 12 and the upper punch 13 becomes small, and if this angle is too large, the upper punch 13 becomes difficult to slide on the tapered portion 122, so neither is preferable.

The alignment of the die 12 and the upper punch 13 will be described with reference to FIG. 5. FIG. 5(a) shows the state in which the lower punch 14 is inserted and the die 12 filled with the powder material M is placed inside the vacuum chamber 10 (more specifically, on the lower spacer 16). The upper punch 13 is attached to the upper spacer 15. After that, as described later in the explanation of the discharge plasma sintering method, when the heating process is performed with the inside of the vacuum chamber 10 in a vacuum atmosphere, the lower electrode 18 is raised by the press mechanism 30. As shown in FIG. 5(b), when the die 12 moves in the direction of the arrow A, if the central axis of the upper punch 13 and the central axis of the inner space 11 are misaligned, a part of the lower end of the upper punch 13 hits a part of the die 12. At this time, since the upper punch 13 is held by the upper spacer 15 so as to be able to move laterally, and the die 12 is provided with a tapered portion 122, as shown in FIG. 5(c), as the die 12 moves further in the direction of arrow A, the upper punch 13 slides along the inclination of the tapered portion 122 with the flange 132 resting on the protruding portions 152a, 153a of the upper spacer 15, and moves in the direction of arrow B. In this way, the upper punch 13 is automatically aligned with the inner space 11 when it is inserted into the inner space 11 in a vacuum atmosphere by the operation of the press mechanism 30.

(heater)
In another embodiment of the present invention, a heater 42 may be provided as a heating unit for heating the powder material M from outside the die 12. The heater 42 has, for example, a ring shape arranged so as to surround the outer periphery of the die 12, and is held by a heater holder 41. The heater 42 can be moved from above inside the vacuum chamber 10 to the side of the die 12 by the operation of a vertical drive unit 43, so that there is no problem with the operability when the die 12 is put in and taken out of the vacuum chamber 10.

[Spark Plasma Sintering Method]
6 is a flow chart 600 showing steps of a spark plasma sintering method according to an embodiment of the present invention. The flow chart 600 shows the steps of setting the die 12 and the upper punch 13, removing impurities, and sintering and cooling the powder material M. Before the flow chart 600, a pre-process and a post-process are performed.

(Pre-process)
First, an operator inserts the lower punch 14 into the inner space 11 of the die 12 outside the vacuum chamber 10, and fills the inner space 11 formed by the die 12 and the lower punch 14 with a required amount of powder material M. In this embodiment, the die 12 is cylindrical, and the lower punch 14 is columnar, but this is not limited thereto, and a die and punch having a shape that matches the desired shape of the sintered body to be produced may be prepared. The powder material M is not limited as long as it can form a sintered body by spark plasma sintering, and for example, metals, alloys, and ceramics such as oxides, carbides, and nitrides may be appropriately used.

(Installation process)
Next, the die 12 filled with the powder material M and with the lower punch 14 inserted therein is placed at a predetermined position on the lower spacer 16 in the vacuum chamber 10 (s601). A thermocouple (not shown) for temperature control is attached to the die 12. The upper punch 13 is attached to the upper spacer 15 as shown in Figures 1 and 5 (s602). At this time, the upper punch 13 is moved laterally and inserted between the hooks 152, 153 so that the flange 132 rests on the protruding portions 152a, 153a of the hooks 152, 153. Finally, the door of the vacuum chamber 10 is closed.

(Impurity Removal Process)
The process of removing impurities preferably includes a non-heating impurity removal process of removing gas present between particles of the powder material M without heating in a vacuum atmosphere, and a heating impurity removal process of removing gas and foreign matter attached to the particles of the powder material M while applying heat to the powder material M in a vacuum atmosphere. First, the door of the vacuum chamber 10 is closed, and then the opening and closing valve 22 is opened and the air in the vacuum chamber 10 is exhausted by the vacuum pump 21 (s603). At this time, the upper punch 13 is not inserted into the inner space 11 of the die 12. By performing exhaust in this manner without inserting the upper punch 13 into the inner space 11 of the die 12, the gas present between particles of the powder material M can be removed (s604: non-heating impurity removal). The degree of vacuum can be appropriately selected depending on the type of powder material M, the target performance and characteristics of the sintered body to be produced, etc., but is typically preferably 10 Pa to 1×10 ⁻³ Pa.

Next, the press mechanism 30 is operated, and the lower electrode 18 is moved upward while maintaining the vacuum inside the vacuum chamber 10, thereby inserting the upper punch 13 into the inner space 11 (s605). At this time, the alignment between the upper punch 13 and the inner space 11 is performed as described above with reference to FIG. 5. When the pressure applied to the powder material M reaches a predetermined pressure, electricity is applied by the upper electrode 17 and the lower electrode 18, and the powder material M is heated until it reaches a predetermined temperature (s607). The pressure applied to the powder material M and the predetermined temperature can be appropriately selected according to the type of powder material M and the target performance and characteristics of the sintered body to be produced. The pressure is preferably 0.1 kN or more, and can be used up to the maximum pressure of the device 1 as necessary. The temperature can be any temperature from higher than room temperature to lower than the sintering temperature as necessary. As an example, the pressure can be 0.1 kN and the temperature can be 150°C.

After heating to a predetermined temperature, the press mechanism 30 is operated to move the lower electrode 18 downward, thereby removing the upper punch 13 from the inner space 11. The heated powder material M is held in this state in a vacuum atmosphere for a predetermined time (s608). The holding time can be appropriately selected according to the type of powder material M and the target performance and characteristics of the sintered body to be produced, and can be 1 to 99 minutes as necessary. As an example, the holding time can be 3 minutes. By holding the heated powder material M in a vacuum atmosphere while the upper punch 13 is not inserted into the die 12, gas and impurities attached to the surface of the particles of the powder material M are removed (s609: Heating impurity removal). As described above, by performing both the non-heating impurity removal process and the heating impurity removal process, a sintered body with high purity and high mechanical strength can be obtained. The heating impurity removal may be performed multiple times as necessary, and in that case, the steps s605 to s609 are repeated multiple times. In the device 1 according to the present invention, the upper punch 13 can be inserted into and removed from the die 12 in a vacuum atmosphere, making it possible to perform the heating and impurity removal process multiple times.

In another embodiment, the impurity removal process may be limited to only thermal impurity removal. In this case, after the upper punch 13 is attached to the upper spacer 15 in s602, the press mechanism 30 is operated to insert the upper punch 13 into the die 12, and at the same time or before or after, the air in the vacuum chamber 10 is exhausted (s606). Thereafter, electricity is passed through the upper electrode 17 and the lower electrode 18 to heat the powder material M until it reaches a predetermined temperature (s607). Furthermore, the upper punch 13 is removed from the inner space 11, and the heated powder material M is held in this state in a vacuum atmosphere for a predetermined time (s608), thereby performing thermal impurity removal (s609). In this case, since no non-thermal impurity removal is performed, it is particularly preferable to perform thermal impurity removal multiple times by repeating the steps s605 to s609 multiple times.

In yet another embodiment, the powder material M can be heated by heat from outside the die 12 using the heater 42. In this case, after the non-heating impurity removal process is completed, the heater holder 41 is moved so that the heater 42 is positioned to the side of the die 12, and the powder material M is heated without the upper punch 13 being inserted into the internal space 11, and the powder material M is held in a vacuum atmosphere for a predetermined time with the heating continued or stopped.

(Sintering and cooling process)
After the heated powder material M is held in a vacuum atmosphere for a predetermined time, the press mechanism 30 is operated to move the lower electrode 18 again while maintaining the vacuum inside the vacuum chamber 10, thereby inserting the upper punch 13 into the inner space 11 again (s610). The operation of the press mechanism 30 is performed until the pressure applied to the powder material M reaches a predetermined value. After the predetermined pressure is reached, electricity is passed through the upper electrode 17 and the lower electrode 18 at a predetermined heating rate and sintering temperature for a predetermined sintering time, and sintering is performed (s611). The pressure, heating rate, sintering temperature, and sintering time during sintering may be appropriately selected depending on the type of powder material M, the target performance and characteristics of the sintered body to be produced, and the like. For example, the pressure can be 7 kN, the heating rate 240°C/min, the sintering temperature 500°C, and the sintering time 2 minutes. When sintering is complete, the current is stopped, and the die 12 is left to stand until its temperature drops to the set temperature, after which rapid cooling is performed by blowing cooling gas onto the die 12 as necessary (s612).

(Post-process)
After the cooling of the die 12 is completed, the post-processing is performed. Specifically, it is as follows. First, the vacuum state of the vacuum chamber 10 is released. Next, the press mechanism 30 is operated to move the lower electrode 18 downward, and the die 12, the upper punch 13, and the lower punch 14 are removed from the vacuum chamber 10. At this time, since the flange 132 of the upper punch 13 is placed on the protruding parts 152a, 152b of the upper spacer 15, the upper punch 13 is moved laterally and removed from between the hooks 152, 153. If necessary, a jig is used to remove the upper punch 13 and the lower punch 14 from the die 12, and the sintered body is removed. If necessary, burrs are removed to complete the sintered body.

Example 1
The device according to the present invention can insert and remove the upper punch from the die in a vacuum atmosphere. Therefore, it is possible to perform the thermal impurity removal multiple times. Here, the impurity removal effect when the thermal impurity removal was performed multiple times was confirmed. Three types of sintered samples were prepared using a punch with a diameter of 15 mm and a corresponding die, and the impurity removal effect was compared. The powder material, weight of the sintered body, sintering process conditions, and thermal impurity removal conditions of the three types of samples are as shown in Tables 1 to 3. In the tables, "heated Degas" stands for "heated impurity removal". In samples No. 8 and No. 9, stearic acid powder was assumed to be an impurity and mixed into the powder raw material. The number of times the heated Degas was repeated was two.

The experimental results are shown in Table 4. Figures 7 and 8 are element mapping images of the cut surfaces of the sintered bodies of Samples No. 8 and No. 9, which were cut and polished, and then measured by an EDS (energy dispersive X-ray analyzer).

Comparing sample No. 8 and sample No. 9, the sintered weight of sample No. 8 was lighter, and the inside of the vacuum chamber turned white overall after sintering of No. 8. This is thought to be because the stearic acid was vaporized by the thermal impurity removal and adhered to the cold inner walls of the vacuum chamber and to the viewport glass of the vacuum chamber. Also, the EDS observation results show that there is less C (carbon) remaining in the cross section of the sample in sample No. 8 (Figure 7) than in sample No. 9 (Figure 8). From these findings, it is thought that impurities in the powder material can be removed before sintering by thermal impurity removal, making it possible to increase purity.

REFERENCE SIGNS LIST 1 Spark plasma sintering apparatus 10 Vacuum chamber 11 Inner space 12 Die 121 Body 122 Tapered portion 13 Upper punch 131 Body 132 Flange 14 Lower punch 15 Upper spacer 151 Body 152, 153 Hook 152a, 153a Projection portion 16 Lower spacer 17 Upper electrode 18 Lower electrode 20 Exhaust mechanism 21 Vacuum pump 22 Opening and closing valve 30 Press mechanism 40 Heating mechanism 41 Heater holder 42 Heater 43 Vertical drive unit M Powder material

Claims (4)

A spark plasma sintering method for sintering a powder material filled in an inner space of a cylindrical die by applying electric current while pressing the powder material with a punch in a vacuum atmosphere, comprising:
a vacuum process for creating a vacuum atmosphere around the die filled with the powder material;
a heating step of heating the powder material in a vacuum atmosphere ;
a holding step of holding the heated powder material in a vacuum atmosphere with the punch not inserted into the internal space;
and a sintering step of inserting the punch into the internal space after the holding step in a vacuum atmosphere and applying pressure to the powder material with the punch while passing an electric current through the powder material. The spark plasma sintering method according to claim 1, wherein the heating step includes inserting the punch into the internal space, heating the powder material by passing current through the punch, and removing the punch from the internal space. The vacuum process includes creating a vacuum atmosphere around the die filled with the powder material while the punch is not inserted into the internal space of the die.
The spark plasma sintering method according to claim 1 or 2. The spark plasma sintering method according to claim 1 , wherein the heating step comprises heating the powder material by externally heating the die.

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