FUNCTIONALLY GRADIENT MATERIAL AND METHOD OF MANUFACTURING SAME

Abstract: A functionally gradient material disclosed is the one obtained by cast-forming slurry containing therein groups of particles of a plurality of types, which are different in specific gravity and sintering the cast slurry, said groups of particles of the plurality of types containing at least a first group of particles and a second group of particles, said first group of particles being a group of non-metallic particles containing one or more groups selected from groups consisting of an oxide, a carbide, a nitride and an acid nitride, which have a specific gravity within the range of 5 - 7 and have the largest particle diameter less than the deflocculation limit, and said second group of particles being a metallic group of particles having a specific gravity of about 1.5 times that of said first group of particles or more and have diameters scattering on both sides of the deflocculation limit. A method of manufacturing this functionally gradient material comprises the steps of: providing the slurry containing the above groups of particles of the plurality of types and/or single slurries of a plurality of types, respectively; suitably supplying the same to a porous mold; and forming a wall forming portion in said porous mold in consideration of the adsorption effect of the porous mold and the deflocculation effect in the slurries of various particles, the influence of gravity or the like.

Description: Technical Field The present invention relates to a process for the preparation of a sealing cap for a discharge lamp bulb, using a gradient function material. US 4,602,956 describes composite cermets which are useful as end closures for metal halide discharge lamps and which have a central core of a first cermet composition and one or more surrounding layers of different cermet compositions. The composite cermets are formed by a multi-step pressing operation, followed by sintering. US 4,822,312 describes combination sealing discs and electrodes for discharge lamps. The electrodes comprise layers of different composition and are formed by pressing and then sintering layers of powders of different composition. DE-A 2032277 describes a sealing structure for a discharge lamp bulb in which an electrode core is surrounded by rings of material of varying coefficient of thermal expansion. The compositions and hence the characteristics of the prior art sealing caps for discharge lamp bulbs vary in a stepwise manner. This can give rise to problems with peeling of the different layers and consequent deterioration of the seal under thermal stress. Background Art It has heretofore been known to use a gradient function material as a heat resistant material in locations where the difference between inner and outer temperatures is very large, e.g., as a surface layer material for a space shuttle. Conventional gradient function materials have been produced by either making, laminating, pressing, and thereafter firing a number of green sheets which have slightly different compositions, or an evaporation process such as CVD or the like. However, a gradient function material which is produced by laminating green sheets does not have a completely continuous range of compositions, but has compositions varying stepwise transversely thereacross. Therefore, such a gradient function material fails to fully perform the desired function thereof. A gradient function material which is manufactured by an evaporation process such as CVD or the like can have a completely continuous range of compositions. However, since the thickness of a film that can be deposited in one evaporation process is highly small, it is very difficult from the standpoint of cost and technology to obtain a gradient function material having a required thickness. DE 2 517 931 discloses a process for the preparation of a functionally graded material with a continually changing composition from two or more particulate substances of different specific weights. There have been proposed methods of achieving a completely continuous range of compositions and maintaining a desired material thickness as disclosed in Japanese laid-open patent publications Nos. 3-165832 and 3-274105. According to the method disclosed in Japanese laid-open patent publication No. 3-165832, a first slurry containing Ti particles or the like and a second slurry containing SiC particles or the like are prepared, and supplied to a filtering tank which has a filter while continuously varying the mixing ratio of the first and second slurries. When the slurries are drawn by a vacuum pump, a cake having a gradient composition is formed on the filter. The cake is then formed to shape while at the same time it is being dehydrated, after which the shaped cake is fired. According to the method shown in Japanese laid-open patent publication No. 3-274105, a slurry is prepared which contains a plurality of types of particles having different particle size distributions. The slurry is put in a container made of film (corresponding to the filter disclosed in Japanese laid-open patent publication No. 3-165832), and a cake having a gradient composition is formed in the container by centrifugal separation or sedimentation. The cake is then formed to shape while at the same time it is being dehydrated, after which the shaped cake is fired. However, the apparatus which are required are complex and expensive because the vacuum pump and a rotating device for generating the centrifugal force are necessary. The gradient layer that is formed is of an increased thickness of several millimeters as necessitated by the handling of the cake. Therefore, the gradient layer has a large heat capacity and is of a structure vulnerable to a heat shock. FIG. 24 is an enlarged view showing the manner in which particles are attracted when a filter or a container of film is used. Particles 202 are concentrated on holes 201 in a filter 200, creating gaps in regions other than the holes 201. Therefore, a cake that is produced has a water content of 30 % or more. Since such a cake cannot directly be fired, it has heretofore been customary to dehydrate the cake, making the manufacturing process complicated. When a cake is dehydrated and dried, the product tends to crack and shrinks to a very large degree due to dehydration and drying. Therefore, it is necessary to cut out the product after the cake is dehydrated and dried. FIG. 25 is a cross-sectional view of a gradient layer 204 which is formed according to a conventional method. Inasmuch as particles are attracted under suction forces applied in a constant direction in the conventional method, the laminae of the gradient layer 204 are arranged as horizontal stripes, and are liable to peel off because they are superposed too orderly in the transverse direction. Furthermore, gradient function materials manufactured according to the conventional methods have shapes that are limited to simple shapes. Specifically, according to the conventional methods, since water has to be removed from a cake in a pressing step, the gradient layer would be deformed out of an orderly configuration when dehydrated if it were of a complex shape. Consequently, the prior methods proposed in Japanese laid-open patent publications Nos. 3-165832 and 3-274105 suffer the following drawbacks: a) As the vacuum pump and the rotating device for generating the centrifugal force are necessary, the apparatus is complex and expensive. b) The gradient layer which is formed is of a large thickness of several millimeters as necessitated by the handling of the cake. Therefore, the gradient layer has a large heat capacity and is of a structure vulnerable to a heat shock. c) If a filter or a film is used, the water content of a cake is large (30 % or more). Because such a cake cannot directly be fired, it is indispensable to dehydrate the cake, making the manufacturing process complex. d) When a cake is dehydrated and dried, the product tends to crack and shrinks to a very large degree due to dehydration and drying. Therefore, it is necessary to cut out the product after the cake is dehydrated and dried. e) Inasmuch as particles are attracted under suction forces applied in a constant direction, the laminae of the gradient layer are arranged as horizontal stripes, and are liable to peel off because they are superposed too orderly in the transverse direction. f) Gradient function materials manufactured according to the conventional methods have shapes that are limited to simple shapes. Specifically, according to the conventional methods, since water has to be removed from a cake in a pressing step, the gradient layer would be deformed out of an orderly configuration when dehydrated if it were of a complex shape. g) The composition varies such that a certain component either increases or decreases in the transverse direction. Therefore, any gradient function materials which have a composition peak intermediate in their transverse direction cannot be obtained. Disclosure of the Invention The present invention has been made in view of the foregoing problems. It is therefore an object of the present invention to provide a process for the preparation of a sealing cap for a discharge lamp bulb as defined in claim 1. The gradient function material for use in the process of the invention is produced by molding and thereafter firing a slurry which contains a plurality of groups of particles having different specific gravities, characterized in that the plurality of groups of particles include at least a first group of particles and a second group of particles, the first group of particles comprises a group of nonmetal particles being made of one or more materials selected from the group consisting of alumina, zirconia, magnesia, silica, silicon carbide, titanium carbide, silicon nitride, and AlON, and the second group of particles may comprise a group of particles which have a specific gravity which is about 1.5 times the specific gravity of the first group of particles or greater, the metal particles being made of nickel, tungsten, tantalum, chromium, molydenum or an alloy containing at least one material selected from the group consisting of nickel, tungsten, tantalum, chromium or molybdenum, and wherein the first group of particles have a maximum particle diameter such that they are deflocculated in the slurry, and the second group of particles has particle diameters such that some of the particles are deflocculated and the remaining particles are sedimented in the slurry. The gradient function material may be manufactured by the steps of preparing a first slurry containing at least a group of small-specific-gravity particles having a small specific gravity and a second slurry containing at least a group of large-specific-gravity particles having a large specific gravity, supplying the first slurry or the second slurry singly into a porous mold to form a deposited region in the porous mold, and mixing the first slurry and the second slurry into a mixed slurry and supplying the mixed slurry into the porous mold to allow the particles to be deposited successively from those particles in the mixed slurry which are more susceptible to gravity, onto the deposited region. The gradient function material may also be manufactured by the steps of preparing a first slurry containing at least a group of small-specific-gravity particles having a small specific gravity and a second slurry containing at least a group of large-specific-gravity particles having a large specific gravity, and mixing the first slurry and the second slurry into a mixed slurry and supplying the mixed slurry into a porous mold to allow the particles to be deposited successively from those particles which are more susceptible to gravity. If the porous mold comprises a plaster mold, then a gradient layer having a water content of about 5 %, rather than a cake having a water content of 30 % or more which has heretofore been produced using a filter, is deposited directly in the mold. The gradient function material may be arranged to have such a continuously varying composition that there exists a large-specific-gravity region which is occupied mostly by a group of large-specific-gravity particles having a greatest specific gravity, among the groups of particles, between upper and lower ends thereof in the transverse direction, and the proportion of a group of small-specific-gravity particles having a small specific gravity progressively increases transversely from the large-specific-gravity region toward both ends in the transverse direction. The gradient function material may further be manufactured by the steps of preparing a single slurry containing at least a group of small-specific-gravity particles having a small specific gravity and a mixed slurry containing at least the group of small-specific-gravity particles and a group of large-specific-gravity particles having a large specific gravity, supplying the single slurry into a porous mold to form a deposited region made of the group of small-specific-gravity particles in the porous mold, and supplying the mixed slurry into the porous mold to allow the particles to be deposited successively from those particles in the mixed slurry which are more susceptible to gravity, onto the deposited region made of the group of small-specific-gravity particles, thereby forming a gradient function material forming body. In addition, the step of taking the gradient function material forming body out of the porous mold and removing the deposited region made of the group of small-specific-gravity particles from the gradient function material forming body may be included. If the porous mold comprises a plaster mold, then when a slurry containing a group of particles having a small specific gravity and a group of particles having a large specific gravity is supplied into the plaster mold, a deposited region is formed in contact with a surface of the mold as dominated by being attracted by the plaster mold, but not by a specific gravity and a deflocculated condition, the deposited region having a composition substantially equal to the composition of the slurry. After the deposited region is formed, another deposited region is formed as dominated by the specific gravity and the deflocculated condition. Accordingly, the gradient function material can be manufactured which has a composition peak in an intermediate region thereof. Brief Description of the Drawings FIG. 1(a) is a flowchart of a process of preparing a nickel slurry used in the manufacture of a gradient function material in a method of manufacturing a sealing cap for a discharge lamp bulb according to a first embodiment of the present invention; FIG. 1(b) is a flowchart of a process of preparing an alumina slurry used in the manufacture of a gradient function material in the above method of manufacturing a sealing cap for a discharge lamp bulb; FIG. 2 is a schematic view of a manufacturing apparatus for experimental use in the method of manufacturing a gradient function material according to the first embodiment; FIG. 3 is an enlarged view showing the deposited condition of a deposited layer on a plaster mold, which is produced by the manufacturing apparatus shown in FIG. 2; FIG. 4 is a graph showing a deflocculating effect and a particle size distribution of each of a nickel slurry and an alumina slurry for use in the method of manufacturing a gradient function material according to the first embodiment; FIG. 5 is a cross-sectional view of a gradient layer in a gradient function material that is produced by the method of manufacturing a gradient function material according to the first embodiment; FIG. 6(a) is a graph showing the deposited layer growth rates on a plaster mold of nickel particles when they are in a deflocculated condition and not in a deflocculated condition; FIG. 6(b) is a graph showing the deposited layer growth rate on a plaster mold of alumina particles when they are in a deflocculated condition; FIG. 6(c) is a graph showing the deposited layer growth rate on a plaster mold of a mixture of nickel and alumina particles; FIG. 10 is a comparison table showing characteristics of a gradient function material manufactured by the method of manufacturing a sealing cap for a discharge lamp bulb according to the first embodiment and conventional gradient function materials after they are fired; FIG. 11 is a comparison table showing characteristics of the method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to the first embodiment and conventional methods; FIG. 12(a) is a flowchart of a basic manufacturing process in a method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to a second embodiment of the present invention; FIG. 12(b) is a flowchart of a manufacturing process which employs a sintering assistant different from a sintering assistant used in the manufacturing process shown in FIG. 12 (a); FIG. 13(a) is a graph showing a particle size distribution of Al 2 O 3 used in the manufacturing process shown in FIG. 12(a) ; FIG. 13 (c) is a graph showing a particle size distribution of NiCr used in the manufacturing process shown in FIG. 12(a); FIGS. 14(a) through 14(e) are cross-sectional views showing a process of manufacturing a sealing cap for a metal vapor discharge lamp as a gradient function material according to the manufacturing process shown in FIG. 12 (a); FIG. 15 is an enlarged cross-sectional view of a gradient function material manufactured by the method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to the second embodiment; FIG. 16(a) is a cross-sectional view of a bulb for a metal vapor discharge lamp using the sealing cap manufactured according to the manufacturing process shown in FIG. 12 (a); FIG. 16(b) is a fragmentary cross-sectional view of a sealing cap which is of a type different from the sealing cap shown in FIG. 16(a); FIGS. 17(a) and 17(b) are views showing a method of manufacturing a gradient function material according to a first modification of the method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to the second embodiment; FIG. 18 is a graph showing a composition in the transverse direction of a gradient function material forming body shown in FIG. 14 (b); FIG. 19 is a graph showing a composition in the transverse direction of a gradient function material forming body shown in FIG. 17(b); FIGS. 20(a) and 20(b) are views showing a method of manufacturing a gradient function material according to a second modification of the method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to the second embodiment; FIG. 21 is a graph showing the relationship between the deposited thickness and the electric resistance of the gradient function material forming body shown in FIG. 14(b); FIG. 22 is a graph showing the relationship between the deposited thickness and the Vickers hardness of the gradient function material forming body shown in FIG. 14(b); FIG. 23 is a graph showing the relationship between the deposited thickness and the fracture toughness value of the gradient function material forming body shown in FIG. 14(b) ; FIG. 24 is an enlarged view showing the manner in which a group of particles in a slurry are attracted to a filter used in a conventional method of manufacturing a gradient function material; and FIG. 25 is a cross-sectional view of a gradient layer in a gradient function material produced by a conventional method of manufacturing a gradient function material. Best Mode for Carrying Out the Invention Preferred embodiments of the process according to the present invention will hereinafter be described below with reference to the drawings. FIGS. 1(a) and 1(b) are flowcharts of processes of preparing a nickel slurry and an alumina slurry, respectively, used in the manufacture of a gradient function material in a method of manufacturing a sealing cap for a discharge lamp bulb according to a first embodiment of the present invention. The particle diameters, specific gravities, and other data of nickel and alumina powders as materials are shown in the table 1 below. To prepare a nickel slurry, a mixture of pure water, a deflocculant, a binder, and an antifoam which are weighed as shown in FIG. 1(a) and a nickel powder which is weighed as shown in FIG. 1(a) are mixed in a monopot, after which an antifoam is added to the mixture. The mixture is then deaerated in vacuum and sieved to remove floating substances and contaminants, thus producing a nickel slurry. Similarly, an alumina slurry is produced by weighing, mixing, and processing the materials as shown in FIG. 1 (b). Nickel powder Alumina powder Average particle diameter (µm) 1.2 0.5 Particle size distribution (µm) 0.3 ∼ 9.0 0.1 ∼ 2.0 Shape (-) Nearly sphere Nearly sphere Specific gravity (-) 9.845 3.986 Purity (wt%) 99.50 99.99 Resistivity (Ω·cm/0°C) 6.8 × 10 -6 1.0 × 10 14 Coefficient of thermal expansion × 10 -6 /°C 12.8 7.0 The deflocculant comprises polycarboxylic acid (A6114 manufactured by Toagosei Chemical Industry Co., Ltd.). The binder comprises CMC (SMR-10M manufactured by The Shin-Etsu Chemical Co., Ltd.). The antifoam comprises polyglycol (CE-457 manufactured by Nippon Oils & Fats Co., Ltd). FIG. 2 is a schematic view of a manufacturing apparatus which is used primarily for experimentation and can be used to manufacture a gradient function material. A process for manufacturing a gradient function material using nickel and alumina slurries which have been prepared according to the above processes will be described below. A glass tube 20 coated with was on its inner surface and having a height of 30 mm and a diameter of 40 mm is set on a plaster mold 10 (JIS R9111) held at rest which has a height of 60 mm and a diameter of 100 mm. First, 3 ml. of nickel slurry S1 is poured into the glass tube 20. About 6 minutes later, a deposited layer having a thickness of 0.5 mm is formed on the plaster mold 10. FIG. 3 is an enlarged view showing the deposited condition of the deposited layer. A comparison with the deposited condition shown in FIG. 24 indicates that nickel particles 40 are uniformly deposited on the surface of the plaster mold 10 because the area of pores per unit surface area of the plaster mold 10 is far greater than the filter 200 used in the conventional method. Then, a mixed slurry S2 of 1.5 ml. of nickel slurry and 4 ml. of alumina slurry is poured into the glass tube 20. FIG. 4 is a graph showing a deflocculating effect and a particle size distribution of each of the nickel slurry and the alumina slurry. As shown in the Table 1 above, the nickel slurry has a particle size distribution ranging from 0.3 µm to 9.0 µm, and the alumina slurry has a particle size distribution ranging from 0.1 µm to 2.0 µm. The limit particle diameter within which the deflocculating effect is exhibited is about 6.0 µm. As a result, of the mixed slurry S2 which has been poured into the glass tube 20, the nickel particles which have a particle diameter of 6.0 µm or greater are deposited, and the nickel and alumina particles which have a particle diameter of 6.0 µm or smaller are suspended in the slurry while being repelled by each other. However, the particles in the deflocculated condition are gradually deposited due to gravity. Because the specific gravity of nickel is larger than (more than twice) the specific gravity of alumina, more nickel particles are deposited on the deposited layer of nickel particles which has first been formed on the plaster mold 10 than alumina particles. In this manner, a gradient layer 30 is formed as shown in the cross-sectional view of FIG. 5. FIGS. 6(a) through 6(c) show the relationship between time and deposited thicknesses for the deposited layer growth rates of various particle groups on the plaster mold 10. FIG. 6(a) is a graph showing the deposited layer growth rates of nickel particles. In FIG. 6(a), the curve a indicates the deposited layer growth rate of nickel particles which are not in a deflocculated condition (with no deflocculant), and the curve β indicates the deposited layer growth rate of nickel particles which are in a deflocculated condition (with a deflocculant). In FIG. 6(b), the curve γ represents the deposited layer growth rate of alumina particles which are in a deflocculated condition (with a deflocculant). In FIG. 6(c), curve β + γ represents the actual deposited layer growth rate in the manufacturing method according to the first embodiment. In the first embodiment, the formation of a desired deposited layer is completed in 25 ∼ 30 minutes. The composition of the nickel and alumina particles in the gradient layer 30 continuously varies in a transverse direction thereof as if in transversely superposed corrugated patterns. Specifically, the composition of the nickel and alumina particles varies in a corrugated fashion in a direction normal to the transverse direction and continuously varies or is continuously inclined in the transverse direction. The reason for this is that the order (governed by the specific gravity) of the gradient layer is disturbed because the particles which have a particle size distribution of 6 µm or less are deflocculated and less liable to be governed by the specific gravity. In this embodiment, a first slurry is made of alumina particles and a second slurry is made of nickel particles. However, SiC, zirconia, or the like may be used instead of alumina particles, and W, Cr, Ta, or the like may be used instead of nickel particles. Stated otherwise, a first slurry may be made of a group of particles having a small specific gravity, and a second slurry may be made of a group of particles having a large specific gravity. The first group of particles having a small specific gravity may comprise one or a plurality of groups of nonmetal particles, such as alumina, zirconia, magnesia, silica, silicon carbide, titanium carbide, silicon nitride, and AlON. The second group of particles having a large specific gravity may comprise a group of metal particles of an alloy of nickel, tungsten, tantalum, chromium, or the like, the metal particles having a high melting point, a specific gravity which is 1.5 times that of the first group of particles or greater. FIG. 10 is a comparison table showing characteristics of a gradient function material manufactured by the method of manufacturing a gradient function material according to the first embodiment and conventional gradient function materials after they are fired. FIG. 11 is a comparison table showing characteristics of the method of manufacturing a gradient function material according to the first embodiment and conventional methods. As can be seen from the comparison tables of FIGS. 10 and 11, the gradient function material according to the first embodiment can keep a required thickness in empty regions in the gradient function materials according to the conventional methods. Specifically, the gradient layer produced by vacuum evaporation such as CVD has a maximum thickness of several hundreds µm, and the gradient layer produced by centrifugal separation or solid-liquid separation using a filter has a minimum thickness of several mm. According to the present manufacturing method, it is possible to manufacture a gradient layer having a thickness ranging from several hundreds µm to ten and several mm. With the method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to the first embodiment, the composition of particles making up the produced gradient function material continuously varies in a transverse direction thereof as if in transversely superposed corrugated patterns. Therefore, the gradient function material according to the present invention is less liable to peel off in thin layers and more resistant to thermal stresses than the conventional gradient function materials which are composed of laminae arranged regularly as stripes. In the manufacturing method according to the first embodiment, furthermore, since a gradient layer is formed by being deposited in a plaster mold, any conventional dehydrating step is not necessary. Because no dehydrating step is required, the manufacturing process is simplified, and the gradient layer can keep its shape. In addition, the gradient function material is not limited to a plate shape, but can be formed to any desired shape. In the manufacturing method according to the first embodiment, because no vacuum pump and no rotating device for producing centrifugal forces, or no filter is needed, the apparatus may of a simplified design, which gives an advantage as to the cost. A method of manufacturing a sealing cap for a discharge lamp bulb according to a second embodiment of the present invention and a sealing cap for a discharge lamp bulb manufactured according to such a method will be described below with reference to FIGS. 12(a) and 12(b) through 23. FIG. 12(a) is a flowchart of a basic manufacturing process in a method of manufacturing a gradient function material according to a second embodiment of the present invention. In FIG. 12(a), a group of particles of Al 2 O 3 is used as a group of particles having a small specific gravity, a group of particles of W is used as a group of particles having a large specific gravity, and a sintering assistant of NiCr (80/20) is used. As shown in FIG. 13(a), particles of Al 2 O 3 having a particle diameter of 0.5 µm or less are selected. Since the limit particle diameter within which a deflocculating effect is exhibited with respect to Al 2 O 3 is about 6.0 µm, all particles of Al 2 O 3 having a particle diameter of 0.5 µm or less are deflocculated. The specific gravity of Al 2 O 3 is 3.99 (20°C). Particles having a small specific gravity (small-specific-gravity particles) which may be employed instead of Al 2 O 3 are preferably one or a plurality of groups of nonmetal particles having a specific gravity ranging from 3 to 7 and a maximum particle diameter equal to or smaller than the deflocculation limit, such as of an oxide such zirconia, magnesia, silica, silicon carbide, titanium carbide, silicon nitride, and AlON. Particles of W (tungsten) having a particle diameter ranging from 0.5 µm to 2.3 µm are selected as shown in FIG. 13(b). Since the limit particle diameter within which a deflocculating effect is exhibited with respect to W is about 1.0 µm, some particles of W are deflocculated and the remaining particles of W are sedimented. The specific gravity of W is 19.24 (20°C). Particles having a large specific gravity (large-specific-gravity particles) which may be employed instead of W are preferably metal particles of Mo (molybdenum), Ni (nickel), Ta (tantalum), Cr (chromium), or their alloy, the metal particles having a high melting point, a specific gravity which is 1.5 times that of the small-specific-gravity particles or greater, and particle diameters distributed across the deflocculation limit. Since NiCr is used as a sintering assistant for sintering W at low temperature, it is necessary for the sintering assistant of NiCr to be deflocculated and sedimented in a manner similar to W. Therefore, particles of NiCr having a particle diameter ranging from about 3.5 µm to 5.2 µm are selected as shown in FIG. 13(c). The sintering assistant which may be added instead of NiCr may comprise Ni powder, Cr powder, Co powder, Cu powder, Ti powder, or their mixture, as shown in FIG. 12(b) (the steps following the mixing step in FIG. 12(b) are omitted from illustration as they are the same as those shown in FIG. 12(a)). The sintering assistant should preferably be added in 5 to 50 parts with respect to 100 parts (by volume) of the large-specific-gravity particles such as of tungsten, molybdenum, or the like. The sintering temperature varies depending on the selected sintering assistant. For example, when the sintering assistant is of NiCr and Ni, the sintering temperature ranges from 1300 to 1400°C, and when the sintering assistant is of Cu, the sintering temperature ranges from 1270 to 1320°C. The above starting materials are weighed, and mixed with pure water, a deflocculant, and a binder in a monopot, after which an antifoam is added to the mixture. The mixture is then deaerated in vacuum and sieved by a sieve of 300 mesh to remove floating substances and contaminants. An antifoam is further added to the mixture, and the mixture is deaerated in vacuum, thus producing a mixed slurry S (ceramic/metal mixed slurry) containing a group of small-specific-gravity particles of Al 2 O 3 , etc. and a group of large-specific-gravity particles of W, etc. The mixed slurry S is then poured into a plaster mold, deposited, and dried into a forming body, which is thereafter sintered in a reducing atmosphere thereby producing a gradient function material. Instead of the plaster mold, there may be employed a porous resin mold made of alumina cement, phenolic resin, or the like, or a porous ceramic mold. These molds should have pore diameters which are 2 or 3 times those of particles to be molded (equal to or smaller than those of secondary agglomerated particles). The material of the resin or ceramic mold should be selected which does not contain light elements (Mg or the like) that adversely affects the sintered body. Various steps to be carried out after a mixed slurry S is poured into a plaster mold until a gradient function material is produced will be described below with reference to FIGS. 14(a) through 14(e). FIGS. 14(a) through 14(e) are cross-sectional views showing a process of manufacturing a sealing cap for a metal vapor discharge lamp as a gradient function material according to the process defined in claim 1. As shown in FIG. 14(a), a container coated with was on its inner surface, such as a glass tube 2, is set on a plaster mold 1 (JIS R9111), and the mixed slurry S is poured into the glass tube 2. The region of the poured mixed slurry S which is held in contact with the plaster mold 1 is deposited as dominated by being attracted by the plaster mold 1. As a result, the composition of the deposited region is the same as the composition of the slurry S itself. After the deposition dominated by being attracted by the plaster mold 1 is completed, the mixed slurry S is deposited as dominated by the specific gravity and the deflocculated condition. Specifically, all the particles of Al 2 O 3 are difficult to be deposited as they are deflocculated and suspended while being repelled by each other. Some of the particles of W are deflocculated, and the remaining particles of W are not in a deflocculated condition. Therefore, after particles of W are deposited as being dominated by being attracted by the plaster mold 1, particles of W which are not in a deflocculated condition are sedimented. The particles in the deflocculated condition are gradually deposited due to gravity. At this time, the particle size distribution of the particles of W is greater than the particle size distribution of the particles of Al 2 O 3 and the specific gravity of W is greater than (more than twice) the specific gravity of Al 2 O 3 , as shown in FIGS. 13(a) and 13(b). Therefore, as shown in FIG. 14(b), there is produced a gradient function material forming body 3 with the composition of its gradient layer varying continuously such that there exists a large-specific-gravity region 3a which is occupied mostly by large-specific-gravity particles of W intermediate (in a position close to the bottom in the second embodiment) in the transverse direction (casting direction), and the proportion of small-specific-gravity particles of Al 2 O 3 progressively increases transversely from the large-specific-gravity region 3a. As shown in FIG. 18, which is a graph showing the composition in the transverse direction of the gradient function material forming body 3, the gradient function material forming body 3 exhibits a peak composition of a gradient function material in the large-specific-gravity region 3a. Since an end 3b of the gradient function material forming body 3 which is held in contact with the plaster mold 1 is formed as dominated by being attracted by the plaster mold 1, the composition of the end 3b is close to the composition of the mixed slurry S. The composition of an opposite end 3c is occupied mostly by particles of Al 2 O 3 . Results of experimentation for properties (electric resistance, Vickers hardness, and fracture toughness) along the deposited thickness of the gradient function material forming body 3 are shown in FIGS. 21, 22, and 23. These experimental results indicate that the gradient function material forming body 3 has property peaks corresponding to the peak composition in the intermediate region 3a. As described above with reference to FIG. 25, the gradient pattern of a gradient function material manufactured by the conventional method is such that the stripe laminae of the gradient layer are liable to peel off because they are superposed too orderly in the transverse direction. As with the first embodiment, the gradient function material according to the second embodiment is not produced by deflocculating particle groups in the slurry and sedimenting and depositing the particles as dominated by only the specific gravity. Therefore, the order of the gradient layer (which is dominated by the specific gravity) is disturbed, resulting in a gradient pattern which continuously varies in a transverse direction thereof as if as if in transversely superposed corrugated patterns, as shown in FIG. 15. Since the gradient function material forming body 3 is used as a cap for a metal vapor discharge lamp the portion below the large-specific-gravity region 3a is removed as by grinding after being dried as shown in FIG. 14(c), leaving the gradient function material forming body 3 free of peaks in the varying composition. As shown in FIG. 14(d), the left gradient function material forming body 3 is either temporarily fired for 1 hour or directly processed to shape without being temporarily fired. When the left gradient function material forming body 3 is directly processed to shape, it is shaped to a dimension that matches the diameter of the mouth of a light-emitting tube taking into account the shrinkage which it suffers upon firing. Then, the gradient function material forming body 3 is fired at 1350°C for 6 hours, after which it is processed by HIP in an argon atmosphere. The gradient function material forming body 3 is formed into a cylindrical shape so as to be suitable for use as a sealing cap. Electrode holes 4a, 4b are defined in the gradient function material forming body 3. Then, as shown in FIG. 14(e), an inner electrode 5a is pressed into the electrode hole 4a, and an outer electrode 5b is pressed into the electrode hole 4b, thus producing a sealing cap 6. A metal solder material may be put between the electrodes and the electrode holes when the electrodes are pressed into the electrode holes. FIG. 16(a) shows a bulb 100 for a metal vapor discharge lamp with the sealing cap 6 mounted thereon. The sealing cap 6 has a projection 6a on its distal end which is securely fitted in a recess lla defined in an end of a tubular light-transmissive light-emitting tube 11 which is made of polycrystalline alumina. For securing the sealing cap 6 to the tubular light-transmissive light-emitting tube 11, the portion of the sealing cap 6 to be held against an open end of the light-emitting tube 11 is composed of the end 3c which is occupied mostly by particles of Al 2 O 3 , and the coefficient of thermal expansion of that portion of the sealing cap 6 is selected to be substantially equal to the coefficient of thermal expansion of light-transmissive alumina of which the light-emitting tube 11 is made. The sealing cap 6 is secured to the tubular light-transmissive light-emitting tube 11 by placing a sealing material 12 such as a glass solder or the like between the recess lla and the projection 6a, and heating the sealing material 12 with high-frequency energy or infrared radiation. FIG. 16(b) shows another type of sealing cap 6. In this type, the sealing cap 6 has an outside diameter smaller than the outside diameter of the light-emitting tube 11. The other structural details are the same as those shown in FIG. 16(a). FIGS. 17(a) and 17(b) are views showing a method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to a first modification of the method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to the second embodiment shown in FIGS. 14(a) through 14(e). In the manufacturing method according to the first modification, as shown in FIG. 17 (a), a single slurry S' containing only particles of Al 2 O 3 (particles having a small specific gravity) is supplied into a glass tube 2 on a plaster mold 1 and deposited therein as a deposited layer 3d. Then, as shown in FIG. 17(b), a mixed slurry S of particles of Al 2 O 3 and particles of W is supplied into the glass tube 2, and the particles in the mixed slurry S are deposited successively from the particles of W that are more susceptible to the gravity, onto the deposited layer 3d of A1203. Now, as shown in FIG. 17(b), a gradient composition region whose composition continuously varies transversely is formed on the deposited layer 3d which is composed of 100 % of Al 2 O 3 with a clear boundary defined therebetween. The gradient composition region is formed as dominated by the specific gravity and deflocculation, but not essentially affected by any attraction from the plaster mold 1, and hence is free of any peak compositions. Thereafter, the deposited layer 3d is removed, producing a gradient function material forming body 3 for use as a material of the sealing cap 6. A mixed slurry S may be supplied twice, rather than supplying the single slurry S'. However, since the boundary would be blurred, it is preferable to employ the single slurry S'. FIGS. 20(a) and 20(b) are views showing a method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to a second modification of the method of manufacturing a gradient function material for use in the preparation of a sealing cap for a discharge lamp bulb according to the second embodiment shown in FIGS. 14(a) through 14 (e). In the manufacturing method according to the second modification, as shown in FIG. 20(a), a piston 7 is fitted in a bottom of a glass tube 2 as a container, the piston 7 being slidable along an inner surface of the glass tube 2. Then, s mixed slurry S containing a group of particles having a small specific gravity and a group of particles having a large specific gravity is supplied into the glass tube 2. Thereafter, the mixed slurry S is held at rest for a predetermined period of time until large-specific-gravity particles are of a progressively greater proportion from upper toward lower layers in the composition of the mixed slurry S in the glass tube 2, i.e., the upper layer is occupied mostly by particles of Al 2 O 3 , whereupon a plaster mold 1 is brought into contact with the upper surface of the mixed slurry S, and the piston 7 is elevated to pressurize the mixed slurry S to deposit the slurry on the surface of the plaster mold 1, as shown in FIG. 20(b). In this manner, there is produced a gradient function material whose composition varies only in one direction, without removing the deposited layer 3d as with the first modification. As described above, the present invention offers the following advantages: The gradient function material prepared according to the process defined in claim 1 can keep a required thickness in empty regions in the gradient function materials according to the conventional methods. Specifically, the gradient layer produced by vacuum evaporation such as CVD has a maximum thickness of several hundreds µm, and the gradient layer produced by centrifugal separation or solid-liquid separation using a filter has a minimum thickness of several mm. With the method of manufacturing a gradient function material according to the present invention, it is possible to manufacture a gradient layer having a thickness ranging from several hundreds µm to ten and several mm. The composition of particles making up the gradient function material prepared according to the process defined in claim 1 continuously varies in a transverse direction of the gradient layer thereof as if in transversely superposed corrugated patterns. Therefore, the gradient function material is less liable to peel off in thin layers and more resistant to thermal stresses than the conventional gradient function materials which are composed of laminae arranged regularly as stripes. Since a gradient layer can be formed by deposition using a plaster mold and one type of slurry, any conventional dehydrating step becomes unnecessary, resulting in a simplified process. Because no vacuum pump and no rotating device for producing centrifugal forces, or no filter is needed, the apparatus may of a simplified design, which gives an advantage as to the cost. According to the second embodiment of the present invention, it is possible to form a gradient function material having a composition peak intermediate in the transverse direction. If a gradient function material with no composition peak is required, then the above gradient function material may be processed to shape as an intermediate product. If a single slurry containing a group of particles having a small specific gravity is first supplied into a plaster mold and deposited therein, and then a mixed slurry is supplied into the plaster mold, then since the boundary between a deposited region (with no composition peak) formed from the mixed slurry and a deposited region formed from the single slurry can clearly be distinguished, the deposited region formed from the single slurry can easily be removed subsequently by grinding or the like. Industrial Applicability The process defined in claim 1 is a process for the manufacture of a sealing cap for a discharge lamp bulb, e.g., a sealing cap for the bulb of a metal vapor discharge lamp.

Claims: A process for the preparation of a sealing cap for a discharge lamp bulb, said process comprising forming a gradient function material by preparing a slurry which contains a plurality of groups of particles having different specific gravities, said plurality of groups of particles including at least a first group of particles and a second group of particles, said first group of particles comprising a group of nonmetal particles being made of one or more materials selected from the group consisting of alumina, zirconia, magnesia, silica, silicon carbide, titanium carbide, silicon nitride and AlON; and said second group of particles comprising a group of metal particles having a specific gravity which is about 1.5 times the specific gravity of said first group of particles or greater, said metal particles being made of material selected from the group consisting of nickel, tungsten, tantalum, chromium, molybdenum and alloys containing at least one of nickel, tungsten, tantalum, chromium or molybdenum; wherein the first group of particles have a maximum particle diameter such that they are deflocculated in the slurry, and that the second group of particles has particle diameters such that some of the particles are deflocculated and the remaining particles are sedimented in the slurry, and supplying said slurry into a porous mould to allow the particles to be deposited successively under the effect of gravity to form a gradient function material, and using the gradient function material so formed to manufacture a sealing cap for a discharge lamp bulb. A process as claimed in claim 1 wherein the first group of particles has a maximum particle diameter less than about 6.0 µ m. A process as claimed in claim 1 or claim 2 wherein the first group of particles comprises aluminium oxide particles and the second group of particles comprises nickel particles. A process as claimed in claim 1 or claim 2 wherein the first group of particles comprises aluminium oxide particles and the second group of particles comprises tungsten particles. A process as claimed in any of claims 1 to 4 wherein the porous mould is a plaster mould.

Inventors: HAYASHI KOICHI, ISHIBASHI HIROTAKA, NAGAYAMA HIROYUKI, HAYASHI, KOICHI, ISHIBASHI, HIROTAKA, NAGAYAMA, HIROYUKI, HAYASHI, KOICHI TOTO LTD. 1-1, NAKASHIMA 2-CHOME, ISHIBASHI, HIROTAKA TOTO LTD., NAGAYAMA, HIROYUKI TOTO LTD.
Applicants: TOTO LTD, TOTO LTD.
Application year: 1993
Main classification: C22C29
IPC codes: B22F7, B22F3, B28B1, C22C1, C04B35, B28B17, H01J61