Patent Publication Number: US-2018036801-A1

Title: Three-dimensional manufacturing method

Description:
FIELD 
     Embodiments of the present invention relate to a three-dimensional manufacturing method. 
     BACKGROUND 
     Conventionally, there have been developed various three-dimensional manufacturing methods for manufacturing three-dimensional objects, including a process of forming a powder layer on a manufacturing stage; and a binding process of discharging a binding agent from an inkjet head to a predetermined area of the accumulated powder layer to form a cured layer, for example. By repeating the processes, three-dimensional objects are manufactured. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2010-208069 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     It is difficult to form a uniform powder layer from powder of reduced particle size due to aggregation by particle interaction, which also makes it difficult to manufacture three-dimensional objects in constant quality. To form a desirable powder layer, powder of a particle size of about several tens of micrometers is needed. Thus, large gaps among the particles cause a problem in reduced density and strength of three-dimensional manufactured objects. In the case of using a mixture of two or more materials having different particle sizes and properties, it is difficult to uniformly disperse the materials in the powder layer and provide three-dimensional objects with constant quality. 
     In view of the above, the present invention aims to provide a three-dimensional manufacturing method that can increase the density and the strength of a three-dimensional manufactured object and can manufacture three-dimensional objects with constant quality from a mixture of two or more materials. 
     Means for Solving Problem 
     A three-dimensional manufacturing method according the embodiment comprises: an additive manufacturing process for forming and adding a layer of secondary particles to manufacture a three-dimensional object, the secondary particles obtained by granulating primary particles; and a sintering process for heating the three-dimensional object to produce a sintered compact 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram for explaining the configuration and processes of a three-dimensional manufacturing system according to a first embodiment. 
         FIG. 2  is a schematic sectional view of the configuration of a three-dimensional printer. 
         FIG. 3  is a conceptual diagram for explaining a three-dimensional manufacturing method according to the first embodiment. 
         FIG. 4  is a diagram for explaining a state of secondary particles in pressurization. 
         FIG. 5  is a schematic diagram for explaining the configuration and processes of the three-dimensional manufacturing system according to a second embodiment. 
         FIG. 6  is a conceptual diagram for explaining the three-dimensional manufacturing method according to the second embodiment. 
         FIG. 7  is a perspective view of the appearance of an example of a three-dimensional object having a recess (hollow) manufactured by the three-dimensional manufacturing method according to the second embodiment. 
         FIG. 8  is a diagram for explaining examples of a combination of primary particles. 
         FIG. 9  is a schematic sectional view of a three-dimensional printer according to a modification of the embodiments. 
         FIG. 10  is a perspective view of a main part of a manufacturing tank and a supply device. 
         FIG. 11  is a schematic sectional view of a three-dimensional printer according to a second modification of the embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described with reference to the accompanying drawings. 
     [1] First Embodiment 
       FIG. 1  is a schematic diagram for explaining the configuration and processes of a three-dimensional manufacturing system according to a first embodiment. A three-dimensional manufacturing system  10  according to the first embodiment includes a plurality of (two in  FIG. 1 ) raw material preparation apparatuses  11  (a first raw material preparation apparatus  11 - 1  and a second raw material preparation apparatus  11 - 2 ) and a granulating apparatus  12 . The raw material preparation apparatuses  11  prepare primary particles having different outer shapes. The granulating apparatus  12  mixes the primary particles prepared by the first raw material preparation apparatus  11 - 1  and the primary particles prepared by the second raw material preparation apparatus  11 - 2  with a binder (binding agent) and granulates them to produce secondary particles. 
     The three-dimensional manufacturing system  10  includes an additive manufacturing apparatus  13 , a molding apparatus (TIP: cold isostatic pressing)  14 , and a sintering apparatus  15 . The additive manufacturing apparatus  13  is what is called a three-dimensional printer and forms and adds layers of the secondary particles to manufacture a three-dimensional object. The molding apparatus  14  puts the three-dimensional object manufactured by the additive manufacturing apparatus  13  into a rubber mold and applies isotropic pressure thereto. The sintering apparatus  15  heats and sinters the three-dimensional object after the isotropic pressure application in accordance with a predetermined temperature rising and falling pattern to provide a sintered compact. 
     The raw material preparation apparatuses  11  are described first. The first raw material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  are collectively described because they have the same configuration. 
     First, the materials of the primary particles are described. Examples of the materials of the primary particles include, but are not limited to: oxide materials (metal oxides), such as SiO 2 , alumina (Al 2 O 3 ), zirconia (ZrO 2 ), titanium oxide (TiO 2 ), barium titanate (BaTiC 3 ), lead zirconate titanate (Pb(Zr,Ti)O 3 ), zircon (ZrO 2 .SiO 2 ), cordierite (2MgO.2Al 2 O 3 .5SiO 2 ), forsterite (2MgO.SiO 2 ), mullite (3Al 2 O 3 .2SiO 2 ), and steatite (MgO.SiO 2 ); nitride materials (metal nitrides), such as silicon nitride (SiN), aluminum nitride (AlN), titanium nitride (TiN), and boron nitride (NW; and carbide materials, such as silicon carbide (SiC). 
     The outer shape of the primary particles may be various shapes, such as a spherical shape, an ellipsoidal shape, an acicular shape, and a platy shape. In this case, the primary particles formed by the first raw material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  preferably have different outer shapes. Having the same outer shape, the primary particles preferably differ in particle distribution from each other. This is intended to fill the gaps between the secondary particles for densification by plastically deforming the secondary particles in the pressure molding by the molding apparatus  14 . 
     The first raw, material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  are apparatuses that appropriately add an auxiliary agent, such as a binder, to a powdered ceramic raw material (main material) produced by a solid phase method, a liquid phase method, or a gas phase method and subjects the material to crushing, dispersion, mixing, and other processing. The first raw material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  are, for example, crushing and mixing apparatuses, such as ball mills, bead mills, and jet mills. Furthermore, the first raw material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  are spray driers as needed, for example. 
     Next, the granulating apparatus  12  is described. The granulating apparatus  12  receives a predetermined ratio of the primary particles prepared by the first raw material preparation apparatus  11 - 1  and the primary particles prepared by the second raw material preparation apparatus  11 - 2 , and a predetermined binder as an auxiliary agent, and granulates them to produce the secondary particles. The granulating apparatus is a crushing and mixing apparatus, such as a bail mill, a bead mill, and a jet mill, for example. 
     The following describes a three-dimensional printer  13 A serving as the additive manufacturing apparatus  13 .  FIG. 2  is a schematic sectional view of the configuration of the three-dimensional printer. The three-dimensional printer  13 A includes a treatment chamber  21 , a material tank  22 , a manufacturing tank  23 , a wiper device  24 , an optical device  25 , and a control unit  26 . The treatment chamber  21  maintains a clean space for three-dimensional manufacturing (specially for material oxidation prevention). The material tank  22  accommodates raw materials (secondary particles) of a three-dimensional object. The manufacturing tank  23  is for actual three-dimensional manufacturing. The wiper device  24  supplies the raw materials from the material tank  22  to the manufacturing tank  23 . The optical device  25  emits laser light to each layer of the raw materials (secondary particles), supplied from the wiper device  24  to the manufacturing tank  23 , at a position (pattern) corresponding to each layer of the three-dimensional object corresponding to slice data. The control unit  26  controls the material tank  22 , the manufacturing tank  23 , the wiper device  24 , and the optical device  25 . 
     In the above configuration, the treatment chamber  21  has a sealed space inside. The material tank  22 , the manufacturing tank  23 , the wiper device  24 , and the optical device  25  are arranged at predetermined positions in the treatment chamber  21 . The treatment chamber  21  is supplied with an inert gas, such as nitrogen and argon, front a gas supplier (not illustrated) through a supply port  21 A to maintain cleanliness inside the treatment chamber  21  and exhaust excessive gas components occurring in the three-dimensional manufacturing to outside the treatment chamber  21  through a vent  21 B. 
     The material tank  22  includes a stage  22 A which can be ascended and descended by a hydraulic lifting device  228 . Secondary particles P 20  serving as the raw materials are placed on the stage  22 A. In three-dimensional manufacturing, the stage ascends in each predetermined manufacturing step to move the raw materials of an amount corresponding to a predetermined layer thickness upward in the material tank  22 . 
     The manufacturing tank  23  is supplied with the raw materials from the material tank  22  by the wiper device  24 . The manufacturing tank  23  includes a stage  23 A on which a three-dimensional object is placed. The stage  23 A can be ascended and descended by a hydraulic lifting device  233 . A base plate  230  is placed on the stage  23 A and supports the raw materials and the three-dimensional object as needed. A three-dimensional object MD is additively manufactured by repeatedly forming and adding a layer upon a layer of predetermined thickness. Thus, in the three-dimensional manufacturing, the stage  23 A is descended stepwise in units of the predetermined layer thickness by the hydraulic lifting device  23 B. 
     The wiper device  24  includes a squeezing blade. The squeezing blade is horizontally driven in  FIG. 2 , to supply the amount of the raw materials  22 , corresponding to the predetermined layer thickness moved upward in the material tank  22 , to the manufacturing tank  23  and level them to a uniform thickness. 
     The optical device  25  is placed above the manufacturing tank  23 . The optical device  25  includes an oscillating element, for example. The optical device  25  includes an optical system including a light source, a collimator (collimator lens: convertor lens), a scanner, and a condenser lens (f-θ lens). The light source emits laser light L. The collimator converts the laser light L into parallel light. The scanner includes a galvanometer mirror that deflects the parallel laser light. The condenser lens condenses the laser light (beam) deflected by the scanner on a flat imaging plane for scanning. 
     The control unit  26  is what is called a microcomputer and has a basic configuration including an MPU, a ROM, a RAM, and a communication interface. The control unit  26  controls, via communication lines, the hydraulic lifting devices  225  and  23 B in the material tank  22  or the manufacturing tank  23 , a driving mechanism (not illustrated) of the squeezing blade of the wiper device  24 , and the optical system of the optical device  25 , for example. 
     The molding apparatus  14  includes a high-pressure container and a pressurizing device (not illustrated). The pressurizing device, such as a pump, applies pressure to a liquid (hydraulic medium: pressure transmitting medium) filling the high-pressure container. The sintering apparatus  15  includes a heater for heating and includes an electric furnace. 
     Referring back to  FIG. 1 , a three-dimensional additive manufacturing method according to the first embodiment is described.  FIG. 3  is a conceptual diagram for explaining the three-dimensional additive manufacturing method according to the first embodiment. Upon receiving the respective powdered ceramic raw materials (main materials) and auxiliary agents, such as binders, the first raw material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  crush, disperse, and mix the materials to prepare a dispersed liquid of the primary particles (primary particle manufacture process: primary particle preparation process). 
     Specifically, as illustrated in  FIG. 3( a ) , the first raw material preparation apparatus  11 - 1  produces primary particles P 11  represented by black columnar particles in  FIG. 3 , whereas the second raw material preparation apparatus  11 - 2  produces primary particles P 12  represented by white spherical particles in  FIG. 3 . The sizes of the primary particles P 11  and the primary particles P 12  are several micrometers or less. 
     The primary particles P 11  and the primary particles P 12  may be what is called nanoparticles having a nanometer size. Use of the nanoparticles can further increase reactivity, enabling manufacture of a three-dimensional object with higher density and strength. 
     The two primary particles P 11  and P 12  produced by the first raw material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  are injected into the granulating apparatus  12  at a predetermined ratio. The granulating apparatus  12  receives an injected predetermined binder (predetermined light curing resin, and a sintering aid as needed) as an auxiliary agent to the primary particles P 11  and P 12 , to produce the secondary particles (secondary particle production process). 
     Specifically, as illustrated in  FIG. 3( b ) , the granulating apparatus  12  produces the secondary particles P 20  being granulated powder containing the two primary particles P 11  and P 12  at the predetermined ratio. The size of the secondary particles P 20  is several tens of micrometers, sufficiently large to form a powder layer. 
     The secondary particles P 20  granulated by the granulating apparatus  12  are injected into the material tank  22  of the three-dimensional printer  13 A serving as the additive manufacturing apparatus  13 . Subsequently, the treatment chamber  21  is supplied and filled with an inert gas, such as nitrogen and argon, from the gas supplier (not illustrated) to maintain the cleanliness inside the treatment chamber  21 . 
     The secondary particles P 20  are injected into the material tank  22  up to the uppermost part at a uniform height. In this state, the control unit  26  controls the wiper device  24 . The squeezing blade of the wiper device  24  is horizontally driven in  FIG. 2  under the control of the control unit  26 . The squeezing blade thus supplies, to the manufacturing tank  23 , the secondary particles (raw materials) P 20  of the amount corresponding to a predetermined layer thickness moved upward in the material tank  22  while leveling them to a uniform thickness. 
     The secondary particles P 20  levelled at the predetermined layer thickness in the manufacturing tank  23  are subjected to additive manufacturing by the optical device  25  under the control of the control unit  26  (additive manufacturing process). Specifically, the source of the optical device  25 , placed above the manufacturing tank  23 , generates laser light with the oscillating element and emits the laser light L to the collimator. The collimator converts the laser light L into parallel light and supplies it to the scanner (e.g., a pair of galvanometer mirrors). 
     The parallel laser light L is deflected by the scanner to draw a pattern based on slice data received from outside and reaches the condenser lens. The condenser lens condenses the laser light (beam) L deflected by the scanner on a flat imaging plane, that is, on newly supplied secondary particles P 20  for scanning. As a result, the secondary particles P 20  including the light curing resin are cured into a shape in accordance with the pattern based on the slice data. 
     Upon completion of the curing based on the slice data, the control unit  26  controls the hydraulic lifting device  2 B to descend the stage by the predetermined layer thickness in the manufacturing tank  23 . In parallel with this, the stage in the material tank is ascended by the hydraulic lifting device to be able to supply the secondary particles of the amount corresponding to the predetermined layer thickness to the manufacturing tank  23 . 
     Subsequently, the control unit  26  controls the wiper device  24  to drive the squeezing blade of the wiper device  24  to supply the secondary particles to the top of the manufacturing tank  23  while leveling them to a uniform thickness. The optical device  25  emits the laser light L thereto again for the additive manufacturing of the next layer. 
     Until the entire slice data is processed, The supply of the secondary particles P 20  from the material tank  22  and the curing of the light curing resin included as an auxiliary agent in the secondary particles P 20  in the manufacturing tank  23  are repeated for three-dimensional additive manufacturing in the same manner as described above. Specifically, as illustrated in  FIG. 3( c ) , the secondary particles P 20  are gradually cured, layered, and added into a three-dimensional object MD 1  having a square section as illustrated on the right side of  FIG. 3( c ) . 
     Subsequently, the three-dimensional object MD 1  manufactured by the three-dimensional printer  13 A is put into a rubber container (rubber mold)  41  and placed in a high-pressure container  42 . A pressurizer (not illustrated), applies pressure to a liquid  43  in the high-pressure container  42 , thereby applying isotropic pressure (hydrostatic pressure) to the three-dimensional object MD 1 . 
       FIG. 4  is a diagram for explaining the state of the secondary particles in pressurization. As illustrated in  FIG. 4( a ) , in the three-dimensional object manufactured by the three-dimensional manufacturing apparatus, the spherical-shape (or an ellipsoidal shape) secondary particles P 20 , for example are formed with gaps (holes) therebetween. 
     After the pressurization by the pressurizing device, the secondary particles P 20  are however plastically deformed and densified with no gaps (holes), as illustrated in  FIG. 4( b ) . As a result, in comparison with the three-dimensional object MD 1  before isotropic pressure application, a three-dimensional object MD 2  after the application is shrunk by about a volume corresponding to the gaps between the secondary particles P 20 , as illustrated in  FIG. 3( e ) . 
     Subsequently, the three-dimensional object MD 2  is extracted from the pressurizing device and the rubber container (rubber mold)  41 , and subjected to heating by the sintering apparatus  15  in accordance with a predetermined temperature rising and falling pattern. The three-dimensional object MD 2  is thus sintered into a three-dimensional object MD 3  serving as a sintered compact. More specifically, as illustrated in  FIG. 3( f ) , the three-dimensional object MD 3  as a sintered compact is further shrunk in length to about 70%. The three-dimensional object MD 3  is about 50% to 60% of the three-dimensional object MD 2  after the isotropic pressure application by volume. 
     As described above, according to the first embodiment, the size of the secondary particles P 20  is several tens of micrometers, which enables accurate formation of powder layers for three-dimensional additive manufacturing. Furthermore, the use of the primary particles P 11  and P 12  (e.g., crushed powder), of the secondary particles P 20 , having a size of several micrometers or less can result in reducing manufacturing costs and manufacturing densified three-dimensional objects (three-dimensional structures) having higher density and strength. In place of the isotropic pressure application under the cold condition above, the secondary particles P 20  are applied with pressure by a pressurizing member, such as a press roller and a press plate, in parallel with the three-dimensional manufacturing. 
     [2] Second Embodiment 
     A second embodiment concerns manufacturing a three-dimensional object having a recess (specially, a hollow).  FIG. 5  is a schematic diagram for explaining the configuration and processes of the three-dimensional manufacturing system according to the second embodiment. In  FIG. 5 , same or like components as those in  FIG. 1  are denoted by the same reference numerals, and the detailed explanation thereof is incorporated. 
     A three-dimensional manufacturing system  10 A according to the second embodiment is different from the three-dimensional manufacturing system  10  according to the first embodiment in additionally including a filling apparatus  16  which maintains the shape of the recess (hollow) in a three-dimensional object in the pressurization process and fills the recess (hollow) with a core material that sublimates in the sintering process. The filling process is conducted between the additive manufacturing process by the additive manufacturing apparatus  13  and the pressurization process by the molding apparatus  14 . Examples of the core material include, but are not limited to, a sublimation material, such as naphthalene and anthracene. 
       FIG. 6  is a conceptual diagram for explaining the three-dimensional manufacturing method according to the second embodiment.  FIG. 7  is a perspective view of the appearance of an example of a three-dimensional object having a recess (hollow) created by the three-dimensional manufacturing method according to the second embodiment. As illustrated in  FIG. 7 , a three-dimensional object MD 13  according to the second embodiment has a cuboid shape with a flask-like hollow HL with an opening OP in the top face. 
     In creating the three-dimensional object MD 13  illustrated in  FIG. 6 , the primary particle production process using the first raw material preparation apparatus  11 - 1  and the second raw material preparation apparatus  11 - 2  and the secondary particle production process using the granulating apparatus  12  are the same as those according to the first embodiment. The detailed explanation thereof is therefore incorporated, and the description begins with the additive manufacturing process. 
     The secondary particles P 20  are granulated by the granulating apparatus  12  and injected into the material tank  22  of the three-dimensional printer  13 A serving as the additive manufacturing apparatus  13 . Subsequently, the treatment chamber  21  is supplied and filled with the inert gas, such as nitrogen and argon, from the gas supplier (not illustrated) to maintain the cleanliness inside the treatment chamber  21 . 
     The secondary particles P 20  are injected into the material tank  22  up to the uppermost part of the material tank  22  at a uniform height. The squeezing blade of the wiper device  24  is driven under the control of the control unit  26  to supply, to the manufacturing tank  23 , the secondary particles (raw materials) P 20  of the amount corresponding to the predetermined layer thickness moved upward in the material tank  22  while leveling them to a uniform thickness. 
     The secondary particles P 20  levelled to the predetermined layer thickness in the manufacturing tank  23  are subjected to the additive manufacturing by the optical device  25  under the control of the control unit (additive manufacturing process). Specifically, the secondary particles P 20  including the light curing resin are formed into a shape in accordance with the pattern based on the slice data, that is, a three-dimensional object MD 11  with a cuboid shape having the flask-like hollow HL with the opening OP in the top face, as illustrated in  FIG. 6( c ) . 
     Upon completion of the curing based on the entire slice data, the control unit  26  controls the filling apparatus  16  so as to fill the hollow HL with a core material CR. Specifically, as illustrated in  FIG. 6( d ) , the hollow HL in the three-dimensional object MD 11  is filled with the core material CR up to the opening OP in the top face. 
     Subsequently, the three-dimensional object MD 11  filled with the core material CR is put into the rubber container (rubber mold)  41  and placed in the high-pressure container  42  as illustrated in  FIG. 6( e ) . The pressurizing device applies a pressure to the liquid  43  in the high-pressure container  42 , thereby applying isotropic pressure (hydrostatic pressure) to the three-dimensional object MD 11 . 
     As a result, in comparison with the three-dimensional object MD 11  before the isotropic pressure application, a three-dimensional object MD 12  after the isotropic pressure application is contracted by approximately a volume corresponding to the gaps between the secondary particles P 20  as illustrated in  FIG. 6( f ) . However, the volume of the core material CR hardly changes, so that the shape of the hollow HL needs to be designed considering this property. 
     The three-dimensional object MD 12  is then extracted from the pressurizing device and the rubber container (rubber mold)  41 , and is subjected to heating by the sintering apparatus  15  in accordance with the predetermined temperature rising and falling pattern. The three-dimensional object MD 12  is thus sintered into the three-dimensional object MD 13  serving as a sintered compact. The three-dimensional object MD 12  is rapidly heated until the temperature exceeds the sublimation point of the core material CR. As a result, the core material CR sublimates from a solid into a gas, leaving a hollow HL 1  in the three-dimensional object MD 13  as illustrated in  FIG. 6( g ) . 
     More specifically, the three-dimensional object MD 13  serving as a sintered compact is shrunk in length to about 70%, and becomes approximately 50% to 60% the size of the three-dimensional object MD 12  after the isotropic pressure application by volume. 
     As described above, the second embodiment can provide a three-dimensional object having a recess. As with the first embodiment, the use of the secondary particles P 20  of the size of several tens of micrometers enables accurate formation of powder layers for three-dimensional manufacturing. 
     Furthermore, the second embodiment can use the primary particles P 11  and P 12  (e.g., crushed powder), of the secondary particles P 20 , of several micrometers or less, which can lower manufacturing costs and provide densified three-dimensional manufactured objects (three-dimensional structures) having higher density and strength. 
     [3] Third Embodiment 
     The above embodiments have not described thermal behavior of the primary particles P 11  and P 12  in detail. A third embodiment concerns reducing distortion in a three-dimensional object (three-dimensional structure) in view of the thermal behavior of the primary particles P 11  and P 12 . 
     In this case, a three-dimensional object serving as a sintered compact can be created by the same procedure as that according to the first embodiment and the second embodiment. Alternatively, the molding apparatus (CIP: cold isostatic pressing)  14  that puts a three-dimensional object in a rubber mold and applies isotropic pressure thereto, and the isotopic pressure application process by the molding apparatus  14  may be omitted. 
     More specifically, to form a three-dimensional object before sintering in the third embodiment, the secondary particles P 20  are made of primary particles P 11  and P 12 , one of which is a metallic material and the other is an oxide of the metal. Thereby, the metallic part is oxidized during the heating and the three-dimensional object is sintered with the metal oxide by reaction sintering to at least partially offset a decrease in volume associated with sintering by an increase in volume associated with oxidation of the metallic material. Thereby, distortion in the three-dimensional object can be reduced. 
     Theoretically, the larger the volume expansion of metallic particles as the primary particles when oxidized, the smaller the volume ratio of the metallic particles to the secondary particles P 20  is set. Thereby, the volume shrinkage associated with sintering is at least partially offset by the volume increase associated with oxidation of the metallic material. 
       FIG. 8  is a diagram for explaining an example of combinations of the primary particles. When aluminum (Al) is used as the primary particles P 11 , for example, alumina (Al 2 O 3 ) is used as the primary particles P 12 , and the volume, excluding the binder, of aluminum (metal) in the secondary particles P 20  as composite particles is set to 20% to 70%, as illustrated in  FIG. 8 . Thereby, the volume shrinkage associated with sintering can be at least partially offset by the volume increase associated with oxidation of the metallic material, reducing distortion in the three-dimensional object. 
     If the volume, excluding the binder, of aluminum (metal) in the composite secondary particles P 20  is smaller than 20%, the volume shrinkage associated with sintering becomes too large, whereby distortion of the object cannot be entirely eliminated. By contrast, if the volume, excluding the binder, of aluminum (metal) in the composite secondary particles P 20  is larger than 70%, the volume increase in associated with oxidation of the metallic material becomes too large, whereby distortion of the object cannot be entirely eliminated. 
     Similarly, when zirconia (Zr) is used as the primary particles P 11 , zirconium dioxide (ZrO 2 ) may be used as the primary particles P 12 , and the volume, excluding the binder, of zirconium (metal) in the composite secondary particles P 20  may be set to 20% to 50%, as illustrated in  FIG. 8 . 
     When silicon (Si) is used as the primary particles P 11 , silicon dioxide (SiO 2 ) may be used as the primary particles P 12 , and the volume, excluding the binder, of silicon (metal) in the composite secondary particles P 20  may be set to 10% to 30%, as illustrated in  FIG. 8 . This can reduce distortion in the three-dimensional object. 
     When titanium (Ti) is used as the primary particles P 11 , titanium dioxide (TiO 2 ) may be used as the primary particles P 12 , and the volume, excluding the binder, of titanium (metal) in the composite secondary particles P 20  may be set to 10% to 40%, as illustrated in  FIG. 8 . This can reduce distortion in the three-dimensional object. 
     When hafnium (Hf) is used as the primary particles P 11 , hafnium dioxide (HfO 2 ) may be used as the primary particles P 12 , and the volume, excluding the binder, of hafnium (metal in the composite secondary particles P 20  may be set to 20% to 50%, as illustrated in  FIG. 8 . Thereby, distortion in the three-dimensional object can be reduced. 
     When yttrium (Y) is used as the primary particles P 11 , yttrium oxide (III) (Y 2 O 3 ) may be used as the primary particles P 12 , and the volume, excluding the binder, of yttrium (metal) in the composite secondary particles P 20  may be set to 20% to 50%, as illustrated in  FIG. 8 . Thereby, distortion in the three-dimensional object can be reduced. 
     When nickel (Ni) is used as the primary particles P 11 , nickel oxide (II) (NiO) may he used as the primary particles P 12 , and the volume, excluding the binder, of nickel (metal) in the composite secondary particles P 20  may be set to 20% to 50%, as illustrated in  FIG. 8 . Thereby, distortion in the three-dimensional object can be reduced. 
     When copper (CU) is used as the primary particles P 11 , copper oxide (II) (CuO) may be used as the primary particles P 12 , and the volume, excluding the binder, of copper (metal) in the composite secondary particles P 20  may be set to 20% to 40%, as illustrated in  FIG. 8 . Thereby, distortion in the three-dimensional object can be reduced. 
     When cobalt (Co) is used as the primary particles P 11 , cobalt oxide (II) (CoO) may be used as the primary particles P 12 , and the volume, excluding the binder, of cobalt (metal) in the composite secondary particles P 20  may be set to 20% to 40%, as illustrated in  FIG. 8 . Thereby, distortion in the three-dimensional object can be reduced. 
     When iron (Fe) is used as the primary particles P 11 , iron oxide (III) (Fe 2 O 3 ) may be used as the primary particles P 12 , the volume, excluding the binder, of iron (metal) in the composite secondary particles P 20  may be set to 20% to 30%, as illustrated in  FIG. 8 . Thereby, distortion in the three-dimensional object can be reduced. 
     When tungsten (W) is used as the primary particles P 11 , tungsten oxide (VI) (WO 3 ) may be used as the primary particles P 12 . The volume, excluding the binder, of tungsten (metal) in the composite secondary particles P 20  may be set to 5% to 15%, as illustrated in  FIG. 8 . Thereby, distortion in the three-dimensional manufactured object can be reduced. 
     In this case, the ratio of the primary metallic particles to the secondary particles P 20  does not matter practically as long as it falls within the metal volume ranges illustrated in  FIG. 8 . With an increase in the ratio of the primary metallic particles to the secondary particles P 20 , however, the metallic primary particles are typically larger in size than the primary particles of the metal oxide, so that the density of the three-dimensional object is likely decreased. Thus, the secondary particles P 20  are preferably created at a smaller volume ratio of the metallic primary particles. 
     The above explanation does not include the particle size of the primary particles P 11  and P 12  in detail. Decreasing the particle size of the metal oxide (submicron size) enhances the sinterability, and increases the volume shrinkage associated with sintering. By setting the metal volumes to the higher values in  FIG. 8 , distortion in the three-dimensional object can be inhibited, but the effect is assumed to be small. 
     [4] Modifications of Embodiments 
     The above embodiments have described forming and adding the powder layers of the secondary particles P 20  by a material deposition method for additive manufacturing. Modifications of the embodiments use an additive manufacturing apparatus employing a binder jetting method. 
     [4.1] First Modification 
       FIG. 9  is a schematic sectional view of a three-dimensional printer according to a first modification of the embodiments. A three-dimensional printer  13 B is a three-dimensional manufacturing apparatus that employs the binder jetting. In  FIG. 9 , same or like components as those of the first embodiment in  FIG. 2  are denoted by the same reference numerals, and the detailed explanation thereof is incorporated. 
     As illustrated in  FIG. 9 , the three-dimensional printer  13 B includes the treatment chamber  21 , the manufacturing tank  23 , a supply device  51 , the optical device  25 , and the control unit  26 . In a case where the materials, the secondary particles P 20  are bonded by means other than the laser light L, the three-dimensional printer  13 B does not need to include the optical device  25 . 
     The manufacturing tank  23  includes the stage  23 A, the hydraulic lifting device  23 B, and a peripheral wall  23 D. The secondary particles P 20  as the materials are sequentially supplied onto the top face of the stage  23 A based on slice data. 
     The supply device  51  supplies the secondary particles P 20  above the stage  23 A in the manufacturing tank  2 , and additively forms and bonds layers of the supplied secondary particles P 20  with a binding agent such as an adhesive. The supply device  51  includes an ejector  61 , a mover  62 , a container  63 , and a collector  64 . The ejector  61  ejects the secondary particles P 20  as the raw materials and the binding agent. The mover  62  moves the ejector  61 . The container  63  accommodates the raw materials. The collector  64  collects the raw materials that are not used for manufacturing. 
       FIG. 10  is a perspective view of an essential part of the manufacturing tank and the supply device. As illustrated in  FIG. 10 , the ejector  61  of the supply device  51  includes a holder  71 , nozzles  72 A to  72 E, and tanks  73 A to  73 E. The nozzles  72 A to  72 E are provided integrally with the holder  71 . The tanks  73 A to  73 E correspond to the nozzles  72 A to  72 D. 
     The holder  71  holds the tanks  73 A to  73 B and includes the nozzles  72 A to  72 E on the bottom face corresponding to the tanks  73 A to  73 E. 
     In the above configuration, the tanks  73 A to  73 C may store therein the same secondary particles P 20  or different kinds of secondary particles P 20 , for example. The tank  72 D stores therein a predetermined binding agent and the tank  73 E stores therein a solvent for the binding agent, for example. 
     To simplify the explanation, the following describes the tanks  73 A to  73 C containing the same secondary particles P 20 , for example. 
     The mover  72  includes a rail  81  and a pair of conveyers  82 . The mover  72  moves the ejector  61  in X-axis and Y-axis directions, to move the tanks  73 A to  73 C integrated with the holder  71  of the ejector  61  with respect to the manufacturing tank  23 . 
     The rail  81  is placed above the manufacturing tank  23  and is longer than the manufacturing tank in the Y-axis direction. The holder  71  of the ejector  61  is movable along the rail  81 . The ejector  61  is driven along the rail  81  by a mechanism including various parts such as a motor, a gear, and a belt. The nozzles  72 A to  72 E of the ejector  61  are also moved along the rail  81  to eject the secondary particles P 20  and the binding agent and additively form the layers of the secondary particles P 20  in the manufacturing tank  23 . 
     The collector  64  is connected to the container  63  through a collector tube  86 . The collector  64  suctions not-bonded, powdery secondary particles P 20  and transmits them to the container  63  for collection. 
     Owing to the above configuration, the control unit  26  controls the manufacturing tank  23 , the supply device  51 , and the optical device  25  to additively manufacture a three-dimensional object MD 21  by mutually bonding the secondary particles coated with the binding agent with the optical device. The control unit  26  further controls the collector  64  so as to suction the powdery secondary particles P 20  unused for the manufacturing and transmit them to the container  63  for collection. 
     The three-dimensional object MD manufactured as described above is subjected to the pressurization process, (the filling process), and the sintering process, and formed into a sintered compact, as with the first embodiment and the second embodiment. 
     As described above, the first modification of the embodiments can also reduce manufacturing costs and provide densified three-dimensional objects (three-dimensional structure) having higher density and strength. 
     [4.2] Second Modification 
       FIG. 11  is a schematic sectional view of the three-dimensional printer according to a second modification of the embodiments. In  FIG. 11 , same or like components as those in  FIG. 9  are denoted by the same reference numerals. Similarly to the three-dimensional printer  13 B illustrated in  FIG. 9 , a three-dimensional printer  13 C employs the bond jetting. 
     As illustrated in  FIG. 10 , the three-dimensional printer  13 C includes the treatment chamber  21 , the material tank  22 , the manufacturing tank  23 , the wiper device  24 , and an inkjet manufacturing device  51 . The material tank  22  accommodates raw materials (secondary particles) used to manufacture a three-dimensional object. The manufacturing tank  23  is for actual three-dimensional manufacturing. The wiper device  24  supplies the raw materials from the material tank  22  to the manufacturing tank  23 . The inkjet manufacturing device  51  applies, through an inkjet head, a binding agent RL to each layer of the raw materials (secondary particles), supplied by the wiper device  24  to the manufacturing tank  23 , at a position (pattern) corresponding to each layer of the three-dimensional object corresponding to slice data. 
     The three-dimensional printer  13 C further includes the control unit  26 , a leveling roller  91 , and a press roller  92 . The control unit  26  controls the material tank  22 , the manufacturing tank  23 , and the wiper device  24 . The leveling roller  91  levels the secondary particles P 20 , supplied to the manufacturing tank  23  by the wiper device  24 , to a uniform thickness. The press roller  92  applies pressure to (presses) the top face of the secondary particles P 20  coated with the binding agent RL by the inkjet manufacturing device  51 , thereby increasing the density of the three-dimensional manufactured object. 
     In this case, by the pressure from the press roller  92 , a solution in the binding agent RL applied by the inkjet manufacturing apparatus  51  dissolves the secondary particles P 20 , which facilitates crushing and deforming of the secondary particles P 20  at the time of pressing by the press roller  92  and thereby further increases the density. 
     In the above configuration, the treatment chamber  21  has a sealed space inside. The material tank  22 , the manufacturing tank  23 , the wiper device  24 , and the optical device  25  are arranged at predetermined positions in the treatment chamber  21 . The treatment chamber  21  is supplied with the inert gas, such as nitrogen and argon, from the gas supplier (not illustrated) through the supply port  21 A to maintain the cleanliness inside the treatment chamber. Excessive gas components generated in the three-dimensional manufacturing are exhausted to outside the treatment chamber  21  through the vent  21 B. 
     The material tank  22  includes inside the stage  22 A which can be ascended and descended by the hydraulic lifting device  22 B. The secondary particles P 20  serving as the raw materials are placed on the stage. In the three-dimensional manufacturing, the stage ascends at each predetermined manufacturing step, moving the raw materials of an amount corresponding to a predetermined layer thickness upward in the material tank  22   
     The manufacturing tank  23  includes the stage  23 A, the hydraulic lifting device  23 B, and the peripheral wall  23 D. The secondary particles P 20  being the materials are sequentially supplied to the top face of the stage  23 A based on slice data. 
     The wiper device  24  includes the squeezing blade. The squeezing blade is horizontally driven in  FIG. 2 , to supply, to the manufacturing tank  23 , the raw materials of the amount corresponding to the predetermined layer thickness moved upward in the material tank  22 . The leveling roller  91  levels the secondary particles P 20 , supplied to the manufacturing tank  23  by the wiper device  24 , to a uniform thickness. 
     The inkjet manufacturing device  51  ejects the binding agent RL onto the surface of the secondary particles P 20  supplied to the manufacturing tank  23 , to bind the secondary particles P 20  with each other, and additively forms and bonds the layer of the secondary particles P 20  for the three-dimensional manufacturing. Before the binding agent RL is completely bonded, the press roller  92  applies pressure to (presses) the top face of the secondary particles coated with the binding agent RL by the inkjet manufacturing device  51 , thereby increasing the density of the three-dimensional object. 
     In the above configuration, the inkjet manufacturing device  51  includes the ejector  61 , the mover  62 , the container  63 , and the collector  64 . The ejector  61  ejects the binding agent RL to the secondary particles P 20  supplied to the manufacturing tank  23 . The mover  62  moves the ejector  61 . The container  63  accommodates the raw materials. The collector  64  collects the raw materials (secondary particles) that are not used for manufacturing. 
     The mover  62  includes the rail  81  and the pair of conveyers  82 . The mover  62  moves the ejector  61  in the X-axis and Y-axis directions, to move the tanks  73 A to  73 E integrated with the holder  71  of the ejector  61  with respect to the manufacturing tank  23 . 
     The rail  81  is placed above the manufacturing tank  23  and is longer than the manufacturing tank in the Y-axis direction. The holder  71  of the ejector  61  can be moved along the rail  81 . By driving a mechanism including various parts such as a motor, a gear, and a belt, the ejector  61  is moved along the rail  81 . The nozzles  72 A to  72 E of the ejector  61  are also moved along the rail  81  and eject the binding agent RL to additively form layers of the secondary particles P 20  in the manufacturing tank  23  for the three-dimensional manufacturing. 
     The collector  64  is connected to the container  63  through the collection tube  86  to suction not-bonded, powdery secondary particles P 20  and transmits them to the container  63  for collection. 
     With the above configuration, the control unit  26  controls the manufacturing tank  23 , the supply device  51 , and the optical device  25  for additively manufacturing the three-dimensional object MD 21  by mutually bonding the secondary particles coated with the binding agent RL with the optical device. Furthermore, the control unit  26  controls the collector  64  so as suction the powdery secondary particles P 20  not used for the manufacturing and transmit them to the container  63  for collection. 
     The three-dimensional object manufactured as described above is subjected to the pressurization process, (the filling process), and the sintering process and formed into a sintered compact, as with the first embodiment and the second embodiment. Alternatively, the manufactured three-dimensional object is subjected to the sintering process and formed into a sintered compact. 
     As described above, the second modification of the embodiments can also reduce manufacturing costs and can provide densified three-dimensional objects (three-dimensional structure) having higher density and strength. 
     While certain embodiments of the present invention have been described, these embodiments are given by way of example only and are not intended to limit the scope of the invention. The novel embodiments may be embodied in a variety of other forms, and various omissions, substitutions, and changes may be made without departing from the spirit of the invention. The embodiments and the modifications thereof are included in the scope and the spirit of the invention and in the invention described in the claims and their equivalents. 
     While the above embodiments use the same secondary particles for three-dimensional manufacturing, different kinds of secondary particles can be used for three-dimensional manufacturing, for example.