PRESSURE SELF-COMBUSTION SINTERING METHOD

Abstract: The present invention relates to a pressure self-­ combustion sintering method for obtaining a ceramic sintered body. The method of the present invention is characterised by the step of covering an exterior of an airtight vessel (3), into which a mixture (1) comprising metallic elements and non-metallic elements has been inserted, with a mixture (4) comprising metallic elements and non-metallic elements as the heat source, that is ignition agent, in place of using a filament formed of W, Mo and the like as in known methods.

Description: The present invention relates to a pressure self-combustion sintering method for easily obtaining a ceramic sintered body. A known method of easily obtaining a ceramic sintered body includes a self-combustion sintering method, in which ceramics are simultaneously synthesized and sintered by the use of a reaction heat generated in the synthesis of ceramics from a mixture comprising metallic elements and non-metallic elements, as disclosed in Japanese Patent Laid-Open No. Sho 60-246.270. In particular, in said Japanese Patent Laid-Open it is said that a so-called pressure self-combustion sintering method conducted under pressure by means of a hot isostatic press (hereinafter referred to as HIP) is preferably used. A method of obtaining ceramics reinforced with metallic particles is known from EP-A-0165707. In this method the synthesis of the ceramic and the sintering of a compact body are achieved simultaneously by utilizing the reaction heat generated in the synthesis of the ceramic from metallic and non-metallic elements. The method comprises igniting a part of a powder mixture compact to initiate a chain reaction. Another method is disclosed in Ceramic Engineering and Science Proceedings, Vol. 4, no. 718 (1983) which discloses, at Pages 624-633, a method which simultaneously utilizes reaction exothermicity and high pressure to synthisize high density TiB₂ disks. In order to sinter ceramics by the pressure self-combustion sintering method, a mixture comprising metallic elements and non-metallic elements is inserted into an airtight vessel and then a filament formed of W, Mo or the like as a heat source for starting a reaction is placed within said airtight vessel followed by heating by adding energy to this heat source from outside of the airtight vessel and applying an isostatic pressure to the mixture comprising metallic elements and non-metallic elements to start a synthetic reaction of ceramics from one end of said mixture, whereby carrying out a chain sintering of ceramics by a reaction heat. As a rule, a filament formed of W, Mo or the like is used as the heat source. However, in order to use a HIP method, which is most general as a pressure means in the pressure self-combustion sintering method, an isostatic pressure is applied to the mixture comprising metallic elements and non-metallic elements, so that it is indispensable to insert the mixture comprising metallic elements and non-metallic elements into the airtight vessel. Accordingly, it is indispensable to place the filament formed of W, Mo or the like within this airtight vessel and draw a power supply lead wire formed of the filament made of W, Mo or the like out of the airtight vessel whilst maintaining the airtightness. A tube formed of glass, silica, platinum or the like is generally recommended as a material for use in the airtight vessel. It has been remarkably difficult industrially to draw the lead wire out of the airtight vessel whilst holding the filaments formed of W, Mo or the like airtight within the airtight vessel and it has been said that this is one of the important reasons why the pressure self-combustion sintering method has not been practically used industrially yet. It is an object of the present invention to overcome these problems associated with such known methods. Accordingly, the present invention consists in a pressure self-combustion sintering method, in which ceramics are synthesized and sintered by the use of a reaction heat generated in the synthesis of ceramics from a mixture comprising metallic elements and non-metalic elements under pressure, said mixture being inserted into an airtight vessel, characterised in that said airtight vessel is covered with an outer mixture comprising metallic elements and non-metallic elements as a heat source, that is, ignition agent, for starting a reaction, and heating one end of said outer mixture to start a chain reaction, whereby the generated reaction heat is used. The invention will be described, by way of example with reference to the accompanying drawing, in which: Fig. 1 is a schematic drawing showing one example of HIP apparatus to be used in applying a pressure self-combustion sintering method of the present invention. In the drawing, reference numeral 1 designates reactant consisting of a mixture of metallic elements and non-metallic elements inserted into an airtight vessel 3, and 6 is a carbon case, the heat source, that is, ignition agent 4 being filled between the airtight vessel 3 and the carbon case 6. Reference numeral 5 designates an ignition heater between electrodes 7, 7, and 9 designates HIP vessel, 8 is a HIP heater. HIP apparatus as above filled with reactant and ignition agent functions in the following way: Interior 10 of the HIP vessel is first made high-pressure gas atmospheric with a temperature of 500 - 1200°C, an electric current being applied to an ignition heater 5 from exterior electrode (not shown) via electrodes 7, 7, and then heated. With that heater, ignition agent 4 (for example a composite of Ti and C) is ignited thus starting self-combustion reaction. This reaction finishes in a moment causing high temperature around reactant 1 thus starting self-combustion reaction. As above described, according to the present invention, the self-combustion reaction of the mixture comprising metallic elements and non-metallic elements disposed outside of the airtight vessel, into which the mixture comprising metallic elements and non-metallic elements was inserted, is used as a heat source, that is, an ignition agent, for starting the reaction in the pressure self-combustion sintering method. Accordingly, it goes without saying that it is not required to dispose the filaments formed of W, Mo or the like within the airtight vessel formed of glass and silica with drawing the lead wire out of the airtight vessel while the airtightness is maintained as in the prior arts. In addition, the heating method for starting the self-combustion reaction of the mixture , that is an ignition agent, comprising metallic elements and non-metallic elements disposed outside of the airtight vessel does not require the sintering of the mxiture comprising metallic elements and non-metallic elements, that is, the pressurization, so that an industrially very difficult heat source, such as the disposition of the filaments within the airtight vessel while the airtightness is maintained, is not required but merely direct heating by means of simple filaments or the like is enough. In the above, the heat source, that is, ignition agent, becomes most effective when it is at least as thick as the airtight vessel for heat transmission to the interior through the airtight vessel. Filling density of the ignition agent is considered appropriate when it is 20 - 70% of its theoretical density, because in case it is less than 20%, enough ignition can not be expected and furthermore a mixture of metallic elements and non-metallic elements is hard to be compressed to more than 70%. In order to obtain better release at the time of completion of sintering, it is preferable to coat release agent (2 in Fig. 1) like BN, MoS₂ around the airtight vessel. For a mixture of the metallic elements and non-metallic elements used as the heat source, that is, ignition agent, for starting a reaction, combination of Ti-C, Zr-B, Nb-B, Nb-C, Si-C, Si-N, Ti-N, Al-N, Nb-N may preferably be used. Among these, N is nitrogen gas to keep the interior of HIP high-pressure atmospheric and contributes to the reaction. Accordingly, for a mixture of the metallic elements and non-metallic elements for use in the present invention, a mixture of the metallic elements and gas as above, may also preferably be used. In addition, the present invention relates to the method of synthesizing and sintering an aimed compound by the use of the formation reaction heat of said compound, so that it goes without saying that a starting material, which can generate a sufficient reaction heat, is enough. The synthesis and sintering of ceramics from the mixture comprising metallic elements and non-metallic elements are in particular preferable. At least one kind of metallic elements selected from the group consisting of metallic elements of the groups IIIa, IVa, Va and VIa in the periodic table and at least one of non-metallic elements selected from the group consisting of B, C, N and Si are preferably used because the sufficient formation reaction heat can be obtained. In addition, although a uniaxial press, such as hot press, may be used as the pressing method, the hot isostatic press (HIP) is preferably used industrially. EXAMPLE 1 Ti powders on the market of 55.62 g and B powders on the market of 10.81 g were sufficiently mixed in a ball mill and then molded under pressure in a press followed by enclosing in a vessel made of Pyrex glass in vacuum. This sample was placed in a carbon case and a circumference of said vessel made of Pyrex glass was covered with a powdery mixture comprising Ti powders and C powders at an equal molar ratio. This carbon case was placed in a HIP and then the HIP was evacuated followed by heating an inside of the HIP to 700°C. Subsequently, Ar gas was introduced into the HIP until a pressure of 405,3 x 10⁵ Pa (400 atm). The pressure self-combustion sintering of Ti and B was carried out by electrifying W filaments previously prepared in the Ti and C powders within the carbon case from outside of the HIP. A sintered body having a composition of TiB₂-10wt%Ti and a theoretical density of about 99.8 % was obtained. According to the prior art, it has been required to mount the W filaments on said vacuum vessel by a so-called hermetic seal (glass and a metal are spliced to each other whilst maintaining the vacuum) when the pressed powdery mixture comprising Ti powders and B powders is enclosed in the vessel made of Pyrex glass. Therefore, the yield has been remarkably reduced industrially. For example, of 100 tests merely 92 tests were successful. On the other hand, according to the present invention, the process such as the hermetic seal, which is industrially inferior in yield, is not required, so that of 100 tests 100 tests were successful. EXAMPLE 2 Si powders on the market of 28 g and C powders on the market of 12 g were sufficiently mixed and then the resulting mixture was molded under pressure to prepare pressed powders. These pressed powders were enclosed in a vacuum vessel made of platinum in vacuum and then the circumference of the vacuum vessel was covered with a powdery mixture comprising Ti powders and C powders at an equal molar ratio followed by placing in a carbon case. This carbon case was housed in a HIP and was heated at 1,000°C. Subsequently, Ar gas was introduced into the HIP and the inside of the HIP was pressurized until 456 x 10⁵ Pa (450 atm). Subsequently, the pressure self-combustion sintering was carried out by electrifying W filaments previously prepared in the Ti powders and C powders within the carbon case from outside of the HIP. As a result, a sintered body of SiC having a theoretical density of 99.9 % was obtained. EXAMPLE 3 TiO₂ having an anatase type crystalline structure on the market of 240 g, Al powders on the market of 108 g and C powders on the market of 36 g were sufficiently mixed and then the resulting powdery mixture was molded under pressure to obtain pressed powders. The resulting pressed powders were enclosed in a vessel made of Pyrex glass in vacuum and then the vessel made of Pyrex glass was placed in a carbon case. Subsequently, a circumference of said vessel made of Pyrex glass was covered with a powdery mixture comprising Ti Powders and B powders at a molar ratio of 1 : 2. The carbon case was placed in a HIP and then heated until 880°C followed by introducing Ar gas thereinto until a pressure of 456 x 10⁵ Pa (450 atm). The pressure self-combustion sintering was carried out by electrifying W filaments previously prepared in the powdery mixture comprising Ti powders and B powders within the carbon case from outside of the HIP. As a result, Al₂O₃-TiC composite ceramics having a theoretical density of 97.2 % was obtained. As above described, the use of the method according to the present invention led to the possibility of industrially applying the pressure self-combustion sintering method.

Claims: A pressure self-combustion sintering method, in which ceramics are synthesized and sintered by the use of a reaction heat generated in the synthesis of ceramics from a mixture (1) comprising metallic elements and non-metalic elements under pressure, said mixture being inserted into an airtight vessel (3), characterised in that said airtight vessel (3) is covered with an outer mixture comprising metallic elements and non-metallic elements as a heat source, that is, ignition agent (4), for starting a reaction, and heating one end of said outer mixture (4) to start a chain reaction, whereby the generated reaction heat is used. A pressure self-combustion sintering method as claimed in Claim 1, wherein at least one kind of metal selected from the group consisting of metals of the groups IIIa, IVa, Va and VIa in the periodic table is used as said metallic elements. A pressure self-combustion sintering method as claimed in Claim 1 or 2, wherein at least one element selected from the group consisting of B, C, N and Si is used as said non-metallic elements. A pressure self-combustion sintering method as claimed in Claim 1, 2 or 3, wherein a hot isostatic press (HIP) is used to apply said pressure. A pressure self-combustion sintering method as claimed in any preceding claim, wherein filling density of the heat source, that is, ignition agent (4), for starting a reaction toward the exterior of the airtight vessel (3) is 20 - 70% of its theoretical density. A pressure self-combustion sintering method as claimed in any preceding claim, wherein the heat source, that is, ignition agent (4), for starting a reaction, is at least as thick as the airtight vessel (3).

Inventors: MIYAMOTO, YOSHINARI, NAKATA, HIROHIKO C/O ITAMI WORKS OF
Applicants: OSAKA UNIVERSITY, SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: C04B35
IPC codes: C04B35