Ceramic body and method and apparatus for detecting change thereof

Abstract: To provide diagnosis of service life and maintenance requirements, a sintered ceramic body (1) which in use is subjected to stress and undergoes change with time as a result of the stress is provided with an electrically conductive element (2) in contact with the sintered ceramic material The element (2) has a measurable electrical resistance which increases in dependence on the change of the ceramic body. This permits information to be obtained about said change of said body. Measuring means (6) for the electrical resistance is connected to the conductive element (2). The electrically conductive element (2), as seen in cross-section, may comprise a bundle of elongate members such as fibers or whiskers.

Description: This invention relates to ceramic bodies which in use are subjected to stress, and to methods and apparatus for monitoring changes caused by subjecting ceramic bodies to stress in use. The invention can provide a diagnostic method and apparatus for estimating the remaining useful working life of a ceramic body. One of the biggest problems in application of structural ceramics to actual working parts lies in their low reliability. A ceramic can be superior to a metal in resistance against heat, acid and abrasion, but is less reliable because of low fracture toughness (poor tenacity), creating problems which make it difficult to use the ceramic as structural parts. Many methods of improving toughness (e.g. "Si 3 N 4 -SiC Whisker Based Composite Ceramics," Ceramics No.12, 1983) or a method of confirming the development of cracks ("Non-Destructive Evaluation Technique for Ceramics," Ceramics No.10, 1985) by means of ultra-sonic waves have been proposed to solve these problems. To determine the maintenance period for a gas turbine plant, for example, the service life is calculated (JP-A-59-3335), or the service life of a test piece placed close to the actual machine part is measured for diagnosis (JP-A-57-113360). All these methods fail to diagnose the state of the actual machine part directly, and cannot be called precise life diagnostic methods. The prior art discussed above therefore does not diagnose the service life of the actual machine part directly, leaving much room for improvement in reliability prediction and life diagnosis. This is because, despite particle dispersion and fiber reinforcement, the efforts for improving toughness cannot provide fundamental change of the physical properties of the ceramic; apparent toughness may be improved, but its value may be only one fifth of that of a metal. The method of confirming development of cracks by ultra-sonic waves may allow measurement on the test piece level; however, when it is to be applied to actual parts, it is accompanied by problems for use, such as problems of attaching the sensor to the ceramic parts or additional installation of measuring instruments. Reliability cannot be ensured for use of a ceramic by using the method which does not directly diagnose the state of the machine part. DE-A-3821191 describes a device for measuring pressure difference at a filtering surface in fluid flow. A ceramic disc, having electrical resistances on its surface, is subjected to the pressure difference. Deformation of the disc due to pressure difference is detectable by change of value of the resistances. VDI Zeitschrift, vol. 102, no. 36, 21.12.1960, page 1788 (E. Jung) discloses a strip of what is called colloidal graphite solution is placed on a surface of a concrete body, so that its change of electrical induction indicates crack formation in the concrete. An object of this invention is to provide a ceramic body which in use is subjected to stress and which is adapted to permit accurate monitoring of its condition. A further object of the invention is to provide a method and apparatus for monitoring change in a sintered ceramic body. Another object of the present invention is to provide method and apparatus for judging the service life or remaining useful life of a ceramic part, thus improving maintenance. The invention is based on the concept of a diagnostic conductive element provided in or on the sintered ceramic of an actual machine part. The conductive element should intimately contact the ceramic. In summary, the invention can provide a life diagnostic system to permit judging of the service life of a ceramic structural machine part and determining of maintenance requirement by measuring the change in resistance of the electrically conductive element which may be due to breaking, deformation, temperature, oxidization or corrosion thereof. In one aspect, the invention provides a ceramic body which in use is subjected to stress and undergoes as a result of the stress, as set out in claim 1. In this ceramic body, the electrically conductive element is located at a region which undergoes structural change as a result of said stress, this structural change typically being formation of cracks. The electrically conductive element preferably consists of material having a bulk electrical resistance of less than 1 Ω.cm, and preferably the resistance is uniform along its length. In one form of the invention, the electrically conductive element, as seen in cross-section, comprises a bundle of elongate members selected from fibers and whiskers. Whiskers are short fibers. The fibers or whiskers are gradually broken in use, giving a gradual increase in electrical resistance of the conductive element. Preferably, on average, as seen in cross-section, the electrically conductive element comprises at least ten of these elongate members, more preferably a number in the range 100 to 10 5 . The bundle of elongate members may be entirely made of conductive elongate members or may be a mixture of insulating and conductive fibers or a mixture of insulating and conductive whiskers. The bundle may comprise additionally electrically insulating particles and/or electrically conductive particles. The electrically conductive element preferably has a diameter in the range 100 µm to 1cm, more preferably 100 µm to 1 mm. The electrically conductive element may be embedded in and co-sintered with said ceramic material, or may be formed on the surface of the ceramic material before or after its sintering. The invention can be applied to a wide variety of ceramic parts, for example:- a) a gas turbine part selected from a moving blade, a stationary blade, a transition piece of a combustor, a heat exchanger part, a shroud part, and a duct part, b) a nuclear fission power plant part selected from a shielding material part, a heat exchanger part and a piping part, c) a nuclear fusion plant part which is installed on the side remote from the plasma of a wall of a vacuum containment vessel, d) a part of a magnetohydrodynamic power plant for generating electrical energy by movement of a high-temperature gas, said part comprising an inner wall part of a high-temperature gas exhaust duct, e) an automotive turbocharger rotor part, or f) a tundish of a metal casting machine. In another aspect, the invention provides a method of detecting change of the ceramic sintered body of the invention, as set out in claim 17. The change of the ceramic sintered body detected is typically change of a mechanical property, e.g. strength. The step of assessing the change of the ceramic may be performed by comparing the sensed electrical resistance with at least one predetermined value thereof. The predetermined value of the electrical resistance of said electrically conductive element may be one or more of an initial value of said electrical resistance and a predetermined critical maximum value of said electrical resistance. The invention further provides a method of determining a value of a remaining useful working lifetime of the ceramic body of the invention, as set out in claim 21. In this method, the steps of calculating the value of remaining useful working lifetime may comprise comparing the sensed electrical resistance with a predetermined pattern of variation of the electrical resistance of said electrically conductive element in dependence upon said change of said body. The method of use of the ceramic body of the invention may advantageously include relieving thermal stress in the body by heating the body by electrical resistance heating of the electrically conductive element. In yet another aspect, the invention provides apparatus comprising a ceramic body according to the invention and monitoring means therefor. The monitoring means comprises measuring means for measuring said electrical resistance. The monitoring means preferably comprises judging means for comparing an electrical resistance value measured by the measuring means with at least one predetermined electrical resistance value, and providing an output dependent upon a result of the comparison. The judging means may include comparison means for comparing the value of electrical resistance measured by said measuring means with the predetermined value and providing an output signal when the measured electrical resistance exceeds a predetermined threshold value. The monitoring means may further include means for calculating a value relating to a remaining useful working life of said ceramic body, on the basis of the measured electrical resistance. Suitably the monitoring means includes storage means for storing a predetermined pattern of change of the electrical resistance of the conductive element in dependence on change of said ceramic material, and the judging means includes comparison means for comparing the measured electrical resistance with the predetermined pattern. In the present invention, it is effective to combine, in the electrically conductive element, conductive fibers, whiskers and particles with heat-resistant insulating fibers, whiskers and particles. A combination of two or more types fibers and particles having different elasticity is also effective. It is of particular advantage that the electrically conductive element may also serve to reinforce the ceramic part. In the present invention, change in resistance of the electrically conductive element may be caused by the pulling out, breaking, interfacial contact, deformation, chemical corrosion or temperature change of the material, e.g. fibers of the conductive element. This change may be measured and compared with a threshold value, thereby permitting life prediction, life diagnosis and maintenance diagnosis. For example, cracks occurring in the ceramic will spread into the conductive element, resulting in increased resistance. This value is read and is compared with a threshold value, e.g. by a computer installed outside. If the threshold is exceeded, the stop signal may be issued automatically. This allows the equipment to be stopped, and faulty parts to be replaced, before the ceramic parts are broken. The functional ceramic bodies of the invention can be applied to various types of machine structural parts. When the conductive circuit is installed on various types of machine structural parts, it need not be installed on the whole of such a part; it may be installed only close to the place subjected to the maximum stress or on a surface subjected to wear. Preferably, the element should be mounted close to the surface (5 mm or less from the surface). This is because the ceramic part tends to crack, resulting in linear fracture, so sensitivity to surface crack is very important. For example a crack occurring on the surface gradually cuts the bundle of fibers, resulting in increased resistance. The fibers in the bundle are gradually broken and the resistance is gradually increased by providing not a single fiber but a bundle of long fibers, a bundle of combination of fibers and particles. In the present invention it is also possible to reduce the effect of external thermal stress by using the conductive element as a heater for the ceramic body, to relax thermal stress in the body. The ceramic body can be manufactured by conventional processes such as pressure sintering, reaction sintering, HP (hot pressing) and HIP (hot isostatic pressing). It is possible to use a ceramic having a greater strength when the said ceramic is reinforced by making it complex or by particle dispersion. The ceramic used may be a reaction sintered ceramic comprising one or more of oxide, carbide, nitride and carbonitride. This is because change in dimensions of the reaction sintered ceramic is as small as 1 percent or less when sintered from the molded body to the sintered body; so the electrically conductive circuit can be easily provided in or on the molded body without being deformed or broken, and residual stress does not remain in the conductive element itself. Furthermore, since reaction sintered ceramic is porous, it is possible to coat the surface or impregnate the interior with a substance which provides characteristics required when the body is used as a structural part. CVD (chemical vapor deposition), CVI (chemical vapor implantation), PVD (physical vapor deposition), ion implantation and laser methods are available for coating and impregnation. Thus, the surface of the reaction sintered ceramic provided with the conductive element can be composed of compact materials having excellent resistance against abrasion, heat and corrosion. Furthermore, a combined use with the HIP method makes it possible to compact the material after reaction sintering. One form of reaction sintered ceramic body of the invention can be manufactured as follows: the conductive element comprises metallic powder (Si, Al, etc.) or metallic powder and inorganic compound (one or more of oxide, carbide, nitride or carbonitride) incorporated in or on the ceramic body, and the ceramic body is heated in an atmosphere of reactive gas comprising one or more of oxidizing, carbonizing, nitriding and oxynitriding gas. The metallic powder reacts with the reactive gas to generate the product; by combining the element with the said product, a reaction sintered ceramic can be provided with the conductive element having a porosity of 5 to 30 vol. percent. The conductive element may have one-, two- and three-dimensional orientation. Another manufacturing method for the ceramic body according to the present invention is the green sheet forming method, which is effective to reduce manufacturing cost. The green sheet is formed by a doctor blade, using a slurry comprising the organic binder and ceramic powder, and is cut by punching into a desired shape and sintered. The appropriate thickness of the green sheet is 1 to 5 mm, but depending on the form of the part, any desired thickness can be selected in the forming of the diagnostic conductive element. After the green sheets are stacked and sintered, finishing is done as required. Connection between the conductive element in or on the ceramic and external wiring can be made for example by brazing, laser or ultra-sonic waves. Furthermore, use on the surface of a metallic wire such as steel, tungsten and platinum provides heat resistance. The present invention can provide substantially improved reliability of ceramic parts, and facilitates their application to various types of plant components and machine parts, such as engine parts, by providing a system which judges the condition of the part under the actual operating conditions, and allows calculation of service life and aids maintenance. Embodiments of the invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which:- Fig. 1 is a plan view of a diagnostic ceramic body of the invention which is a first wall part of a nuclear fusion reactor; Fig. 2 is a diagram of an application of the first wall part of a nuclear fusion reactor shown in Fig. 1; Fig. 3 is a graph representing the relationship between neutron irradiation time for the first wall part of a nuclear fusion reactor shown in Fig. 1 and resistance of the conductive element; Fig. 4 is a partial perspective view of wall parts of a nuclear fusion reactor; Fig. 5 is a diagram of a diagnostic ceramic body of the invention which is a gas turbine stationary part; Fig. 6 is a graph representing the relationship between gas turbine operation time and resistance of the conductive element of the ceramic body of Fig. 5; Fig. 7 is a perspective view of a gas turbine blade embodying the invention; Fig. 8 is a perspective view of a gas turbine shroud part embodying the invention; Fig. 9 is a partial perspective view of a gas turbine rotor; Fig. 10 is a diagram of a diagnostic ceramic system of the invention including a corrosion-proof piping part; Fig. 11 is a graph representing the relationship between high temperature corrosion time and resistance of the conductive element of the piping part of Fig. 10; Fig. 12 is a diagram of a diagnostic ceramic system of the invention including another corrosion-proof piping part; Fig. 13 is a graph representing the relationship between chemical (acid, alkali) corrosion time and resistance of the conductive element of the piping part of Fig. 12; Fig. 14 is a diagram showing another process of installation of the electrically conductive element in a ceramic body of the invention; Fig. 15 illustrates process steps in another method of installation of the electrically conductive element in a ceramic body of the invention; Fig. 16 is a diagram of a diagnostic ceramic system of the invention employing a combustor tile; Fig. 17 is a graph representing the relationship between combustor operation time and resistance of the conductive elements in the tile of Fig. 19; Fig. 18 is a perspective partial sectional view of a bearing part embodying the invention; Fig. 19 is a perspective partial view of one form of conductive element used in the invention; Fig. 20 is an explanatory view of another form of conductive element used in the invention; Fig. 21 is an explanatory view of yet another form of conductive element used in the invention; Fig. 22 is a perspective partial view of a further form of conductive element used in the invention; Fig. 23 is an explanatory view of a further form of conductive element used in the invention; Fig. 24 is an explanatory view of a further form of conductive element used in the invention; Fig. 25 is a block diagram of an apparatus including monitoring means embodying the invention; Fig. 26 is a flow chart for a method of calculating service life embodying the invention; and Figs. 27(a), 27(b) and 27(c) are graphs illustrating calculation of a critical period of use of a ceramic part. In the drawings, parts having the same or similar functions in the various embodiments are given the same reference numbers. In all of the following Embodiments, the electrical resistance of the conductive element used for life diagnosis is less than 1 Ω.cm. EMBODIMENT 1 A study has been made on the first wall parts of a nuclear fusion reactor. Such a wall part is shown in Fig. 1 and Fig. 2. To make the ceramic parts constituting the first wall parts of the nuclear fusion reactor, 13 parts by weight of a mixture of polyethylene, stearic acid and synthetic wax is added to 100 parts by weight of metallic Si powder having an average particle diameter of 1 µm, and is kneaded. This material is made into a 2 mm thick, 100 mm square plate 1 by an extruding machine. A composite long fiber (1.5 mm in diameter) comprising carbon fibers (10 µm in diameter, 1000 µm in length) and Al 2 O 3 fibers (5 µm in diameter, 1000 µm in length) is used as a conductive element 2 for life diagnosis on this plate. The mixing ratio of the carbon fiber and Al 2 O 3 fiber used is 50 to 50 vol percent, but is not limited to this value; the carbon fiber to Al 2 O 3 fiber ratio can for example be 30 to 70 or 80 to 20 vol %. The diameter of the element 2 should preferably be 0.1mm or more. The molded plate 1 is heated to 1350°C in a furnace under nitrogen atmosphere at a temperature rise rate of 1°C/min, and held at 1350°C for 1 hour, and cooled in the furnace to obtain a reaction sintered Si 3 N 4 ceramic provided with the electrically conductive element 2 for life diagnosis. The binder is degreased in the temperature rise. A compact BN film 3 0.3 mm thick is formed on the surface of the reaction sintered ceramic 1 by plasma flame spraying to provide a heat resistant layer. Since the surface of the reaction sintered material is porous, BN is formed even in the pores; thus, the film 3 is strongly adhered. Terminals 4 of the element 2 are connected to copper wire 5 by silver brazing, and an external measuring device 6 is connected. Such ceramic parts are used as the first wall parts of a nuclear fusion reactor. In use the BN film 3 is damaged by neutron irradiation and change in resistance of the element 2 is observed as shown in Fig. 3, demonstrating that the life diagnosis of the first wall parts of the nuclear fusion reactor is possible. This change in resistance may be fed to a nuclear power plant control system. When this resistance has exceeded a specified threshold, the plant may be automatically stopped to allow maintenance and inspection. Si 3 N 4 , Si 2 N 2 O, AlN, Al 2 O 3 , ZrO 2 , B 4 C, BN and mullite powder and whiskers may be mixed with the Si powder used in Embodiment 1 to manufacture composite ceramics (Si 3 N 4 /Si 3 N 4 , Si 3 N 4 /Si 2 N 2 O, Si 3 N 4 /AlN, Si 3 N 4 /Al 2 O 3 ,Si 3 N 4 /ZrO 2 , Si 3 N 4 /B 4 C, Si 3 N 4 /BN, Si 3 N 4 /mullite, Si 3 N 4 /SiC, etc.) which can be provided the conductive element for life diagnosis, by a reaction sintering method. Ceramic molding is possible in many ways to meet the requirements of the various shapes of the parts of the present invention, such as the doctor blade method, powder molding, CIP, injection molding, cast molding, extrusion, etc. The binders used for molding can be as generally used for molding ceramics, including high-molecular weight materials such as polyvinyl butyral and polyethylene, silicone series compound, and polysilane series compounds. Degreasing methods for these binders need not be specified here in particular; they can be degreased by controlling the temperature rise speed at the time of sintering. EMBODIMENT 2 Using metallic Al powder in place of metallic Si powder in Embodiment 1, it is possible to make an AIN ceramic provided with a conductive element for life diagnosis by reaction sintering, and this may be used in the same way for the first wall parts of the nuclear fusion reactor. EMBODIMENT 3 A further wall part for a nuclear fusion plant was made as follows. 13 parts of a mixture of polyethylene, stearic acid and synthetic wax is added to 97 parts by weight of SiC powder having an average particle diameter of 0.1 µm, 1 part by weight of BeO as sintering agent and 2 parts by weight of AlN, and the whole mixture is kneaded to produce a material, which is shaped into in a 2 mm-thick, 100 mm-square plate by an extruding machine. A composite long fiber (2.5 mm in diameter) comprising carbon fibers (100 µm in diameter, 1000 µm in length) and Al 2 O 3 fibers (100 µm in diameter, 1000 µm in length) is formed to make a conductive element 2 for life diagnosis 2 (Figs. 1 and 2) by molding on this plate. The mixing ratio between carbon and Al 2 O 3 fibers used for the element 2 was 70 to 30 vol percent. The molded plate 1 is heated at a temperature rise rate of 1°C/min. up to 2150°C in a furnace in an atmosphere of argon, and held for three hours at 2150°C. It is then cooled in the furnace, to obtain an atmospheric pressure sintered SiC ceramic body 1 provided with the element 2. To provide heat resistance, a compact 0.3 mm-thick BN film 3 was formed on the surface carrying the element 2 by plasma spraying. Copper wiring 5 was silver-brazed on terminals 4 of the element 2 and was connected to external measuring instrument 6 it was then connected to a nuclear power plant diagnostic system. EMBODIMENT 4 Fig. 4 shows the first wall of the nuclear fusion reactor, wherein reactor wall ceramic plates 13 are manufactured in the same way as the plates in Embodiment 3. Each ceramic plate 13 is silver-brazed to a substrate 20 of austenite series stainless steel via a graphite substrate 15 containing carbon fiber whose coefficient of thermal expansion is similar to that of the ceramic plate. A metallic coating layer 16 is present on the steel substrate 20. The substrate 20 is provided with cooling channel 14 in which water flows for cooling. The element 2 is located inside the BN film which is on the outer surface of the plates 13. The element 2 is extended to the substrate 20 and is connected to the copper wire 19. Al 2 O 3 protective pipes 18 cover the connections between the copper wire and element 2. The BN film of the ceramic plates 13 is damaged by neutron irradiation and the change in resistance appearing in the element 2 is as illustrated in Fig. 3. EMBODIMENT 5 Here the stationary blade part of a gas turbine in a thermal power plant system is studied. To manufacture the material for ceramic parts constituting the stationary blade parts for gas turbine, 108 parts by weight of the binder comprising 6 parts by weight of polyvinyl butyral, 24 parts by weight of trichloroethylene, 32 parts by weight of tetrachloroethylene, 44 parts by weight of n-butyl alcohol and 2 parts by weight of butylphthalyl glycolic acid is added to 100 parts by weight of metallic Si powder having an average particle diameter of 0.5 µm, and the mixture is kneaded by a ball mill for 24 hours, to make a slurry. The slurry is subjected to vacuum degassing to remove gas bubbles, and cast into a rubber mold for the stationary blade of the gas turbine by the mold casting method. Then it is dried for ten hours to make a molded body. In the molding, a composite fiber (3 mm in diameter), consisting of 70 vol% of carbon fiber (100 µm in diameter) and 30 vol% of SiC fiber (100 µm in diameter) is formed for a conductive element in the rubber mold according a specified pattern, and the slurry is cast around it. This molded body is heated at a temperature rise rate of 1°C/min. up to 1350°C in a furnace with a nitrogen atmosphere, and held at 1350°C for 1 hour and then cooled in the furnace to obtain a reaction sintered Si 3 N 4 ceramic body 1 (Fig. 5) provided an element 2 for life diagnosis 2. The binder is degreased during the temperature rise. To provide heat resistance, a 30 µm-thick compact ZrO 2 film is formed on the surface of the reaction sintered ceramic by plasma spraying. Tungsten wires 5 are brazed on terminals of the element 2, and connected to an external measuring instrument 6, which is connected to a thermal power plant diagnostic system. In use of this stationary blade part of a gas turbine, the element 2 becomes increased in resistance, typically as shown in Fig. 6, revealing that the life diagnosis was possible. This diagnosis provides substantially improved reliability of the ceramic parts. The change in resistance may be fed to the thermal power plant diagnostic system, which issues a shut down signal if the resistance value has exceeded a specified threshold value, allowing maintenance and inspection. Fig. 6 shows how the resistance varies with time of operation. The initial state of the ceramics is maintained for about 500 hours. After that the ceramic becomes damaged as cracks are generated due to stress, and these cracks progress into the interior of the ceramic body due to fatigue. The element 2 becomes thinner, due to this cracking, and its resistance increases gradually and irreversibly. Fig. 6 shows a critical use period at which the apparatus is stopped, before the ceramic body is destroyed. When the resistance change pattern of Fig. 6 is known, it is possible to determine the critical resistance level related to the desired critical period. This critical period is determined according to the size of the ceramic component, the position of the diagnosis element, the resistance of the diagnosis element and the safety coefficient. Further, as the increase in the resistance is predictable, it is possible to lengthen the allowed residual life. To make the film providing a compact dense surface of the reaction sintered ceramic, the surface layer may be a single layer or multi-layer film of SiC, BN, TiN, ZrO 2 , diamond, ZrB 2 or the like, depending on the conditions and purpose of use. EMBODIMENT 6 Using metallic Al powder in place of metallic Si powder in Embodiment 5, there is made an AlN ceramic blade part provided with a conductive element for life diagnosis by the reaction sintering method, and this is applied to a stationary blade part for a gas turbine. The sintering process involves heating at 1°C/min to 1400°C and holding at 1400°C in a nitrogen atmosphere. The pattern of change in resistance shown in Fig. 6 was observed in operation before the ceramic part was damaged. Furthermore, Si 3 N 4 , Si 2 N 2 O, AlN, Al 2 O 3 , ZrO 2 , B 4 C, BN and mullite powder and whiskers may be mixed with the the Al powder to manufacture composite ceramics (AlN/Si 3 N 4 , AlN/ Si 2 N 2 O, AlN/AlN, AlN/Al 2 O 3 , AlN/ZrO 2 , AlN/B 4 C, AlN/BN, AlN/mullite, AlN/SiC, etc.) provided with electrically conductive elements for life diagnosis, by the reaction sintering method. EMBODIMENT 7 To make a moving blade part (of Fig. 7) of a gas turbine in a thermal power plant system, 108 parts by weight of a binder comprising 6 parts by weight of polyvinyl butyral, 24 parts by weight of trichloroethylene, 32 parts by weight of tetrachloroethylene, 44 parts by weight of n-butyl alcohol and 2 parts by weight of butylphthalyl glycolic acid is added to 96 parts by weight of SiC powder having an average particle diameter of 0.5 µm and 4 parts by weight of BeO powder having an average particle diameter of 0.2 µm. The mixture is kneaded in a ball mill for 24 hours, to be made into slurry. The slurry is subjected to vacuum degassing to remove gas bubbles, and is cast into a rubber mold for the moving blade of the gas turbine, by the mold casting method. Then it is dried for ten hours to give a molded body. A composite conductive fiber 3 mm in diameter is formed from 80 vol% carbon fibers 6 µm in diameter and 20 vol% SiC fibers 10 µm in diameter, to form an electrically conductive diagnosis element, and arranged in the rubber mold according a specified pattern, and the slurry is cast around it. The molded body is heated at a temperature rise rate of 5°C/min. up to 2250°C in a furnace of argon atmosphere, held for one hour at 2250°C and then cooled in the furnace to obtain the SiC ceramic blade part (Fig. 7) provided with the element 2 for life diagnosis. The binder is degreased during the temperature rise. To provide heat resistance, a compact µm-thick ZrO 2 film is formed on the surface of the ceramic by plasma spraying. Tungsten wires 5 in insulating sleeves 18 are brazed on the terminals of the element 2 which extends to the turbine disk, and are connected to an external resistance measuring instrument. This is connected to the thermal power plant control system. This element 2 is used in the way already discussed, and demonstrates that the life diagnosis is possible. Fig. 7 shows the blade 21, dovetail 22 and insertion section 23. The element 2 need not be installed over the whole of the machine structural part; it may be installed only close to the place A subjected to the maximum stress. Furthermore, the element should be mounted close to the surface (5 mm or less from the surface). This is because the ceramic parts tend to crack, resulting in linear fracture, so sensitivity to surface crack is very important. A crack occurring on the surface gradually cuts the bundle of fibers, resulting in increased resistance. EMBODIMENT 8 To manufacture a shroud part 30 (Fig. 8) of a gas turbine, 108 parts by weight of a binder comprising 6 parts by weight of polyvinyl butyral, 24 parts by weight of trichloroethylene, 32 parts by weight of tetrachloroethylene, 44 parts by weight of n-butyl alcohol and 2 parts by weight of butylphthalyl glycolic acid is added to 95 parts by weight of Si 3 N 4 powder having an average particle diameter of 0.1 µm, 3 parts by weight of sintering agent Y 2 O 3 and 2 parts by weight of AlN, and the mixture is kneaded by a ball mill for 24 hours, to be made into slurry. The slurry is subjected to vacuum degassing to remove gas bubbles, and is cast into a rubber mold for the shroud segment part for a gas turbine. Then it is dried for ten hours, to give a molded body. A composite C/Al 2 O 3 fiber (2 mm in diameter) of C fibers 6 µm in diameter and Al 2 O 3 fibers 10 µm in diameter is arranged as a conductive element for life diagnosis in the rubber mold, according to a specified pattern, and the slurry is cast around it. The molded body was heated at the temperature rise rate of 5°C/min. up to 1750°C in a furnace of nitrogen atmosphere and was held for three hours. It was then cooled in the furnace to obtain an atmospheric pressure sintered Si 3 N 4 ceramic shroud part (Fig. 8) provided with an element 2 for life diagnosis 2. The binder can be degreased during the process of temperature rise. To provide heat resistance, a compact 10 µm-thick ZrO 2 film is formed on the surface of the Si 3 N 4 ceramic by plasma spraying. Copper wires 5 are silver-brazed on the terminals of the element 2 which extends to the exterior of the turbine casing, and is connected to an external resistance measuring instrument 6, which is connected to the thermal power plant control system. The element 2 and copper wire are insulated by the alumina protective tubes 18. Ceramic parts as shown in Fig. 8 are applied as the shroud parts of the gas turbine, and a pattern of change in resistance in use as shown in Fig. 5 was observed before the ceramic parts were damaged. Fig. 9 shows a rotary portion of a gas turbine with the built-in ceramic shroud part 30, casing 31, moving blade 32 and turbine disk 33. EMBODIMENT 9 To manufacture another stationary blade part of a gas turbine, similar in shape to that of Fig. 5, 108 parts by weight of a binder comprising 6 parts by weight of polyvinyl butyral, 24 parts by weight of trichloroethylene, 32 parts by weight of tetrachloroethylene, 44 parts by weight of n-butyl alcohol and 2 parts by weight of butylphthalyl glycolic acid is added to 95 parts by weight of Si 3 N 4 powder having an average particle diameter of 0.1 µm, 3 parts by weight of sintering agent Y 2 O 3 and 2 parts by weight of AlN, and the whole mixture is kneaded by the ball mill for 24 hours, to be made into slurry. The slurry is subjected to vacuum degassing to remove gas bubbles, and is cast into a rubber mold for the stationary blade part of the gas turbine, according to the mold casting method. Then it is dried for ten hours, to make a molded body. A composite SiC/Al 2 O 3 fiber (2 mm in diameter) consisting of 70 vol% SiC fiber (50 µm in diameter) and 30 vol% Al 2 O 3 fiber (70 µm in diameter) is installed to make a conductive element for life diagnosis in the rubber mold according to a specified pattern, and the slurry is cast around it. The molded body is heated at a temperature rise rate of 5°C/min. up to 1750°C in a furnace in a nitrogen atmosphere and is held at 1750°C for three hours. It is then cooled in the furnace, to obtain an atmospheric pressure sintered Si 3 N 4 ceramic including a conductive element whose resistance can be measured. The binder is degreased during the process of temperature rise. To provide heat resistance, a compact 10 µ-thick ZrO 2 film is formed on the surface of the sintered Si 3 N 4 ceramic by plasma spraying. Copper wire is silver-brazed on the terminals of the conductive element to connect it to an external resistance measuring instrument 6. This is connected to the thermal power plant diagnostic system. Change in resistance in use as shown in Fig. 6 is observed before the ceramic part is damaged. EMBODIMENT 10 To manufacture a ceramic corrosion-resistant piping part 17 (Fig. 10) of a thermal power plant, 108 parts by weight of a binder comprising 6 parts by weight of polyvinyl butyral, 24 parts by weight of trichloroethylene, 32 parts by weight of tetrachloroethylene, 44 parts by weight of n-butyl alcohol and 2 parts by weight of butylphthalyl glycolic acid is added to 97 parts by weight of SiC powder having an average particle diameter of 0.1 µm, 1 part by weight of sintering agent BeO and 2 parts by weight of AlN. This mixture is kneaded by a ball mill for 24 hours, to be made into slurry. The slurry is subjected to vacuum degassing to remove gas bubbles, and is cast into a porous mold (of plaster) to make the 10 mm-thick pipe having a diameter of 150 mm, using a centrifugal molding machine. A composite fiber (3 mm in diameter) consisting of 50 vol percent of carbon fiber (60 µm in diameter) and 50 vol percent of Al 2 O 3 fiber (100 µm in diameter) is formed to make a conductive element for life diagnosis on the porous mold surface according to a specified pattern, and the slurry is cast around it. After wax is removed from inside the molded body, the body is treated at the temperature of 2100°C in an atmosphere of argon for three hours. This causes the conductive element 2 for life diagnosis to be installed in the pipe part 17. Such ceramic parts 17 are applied as the corrosion-resistant piping parts of the thermal power plant, with the result that patterns of change in resistance shown in Fig. 11 are observed before the ceramic parts were severely damaged by corrosion. This shows that the life diagnosis is possible in such a case. This diagnosis provides substantially improved reliability of the ceramic parts. EMBODIMENT 11 Piping parts as shown in Fig. 12 are made by a process similar to that of Embodiment 10, but with a plurality of individual conductive elements 2 embedded in the ceramic body 17a, as shown in Fig. 12 and all connected to resistance measuring device 6 which measures their electrical resistances sequentially. In a chemical plant, such piping parts may be subjected to corrosion by acid or alkali, which causes gradual irreversible change as shown in Fig. 13 in the electrical resistance of the elements 2, to allow life-time diagnosis. The several elements 2 also allow the location of a corrosion-affected region to be determined. EMBODIMENT 12 To manufacture another corrosion-resistant piping part embodying the invention, 108 parts by weight of a binder comprising 6 parts by weight of polyvinyl butyral, 24 parts by weight of trichloroethylene, 32 parts by weight of tetrachloroethylene, 44 parts by weight of n-butyl alcohol and 2 parts by weight of butylphthalyl glycolic acid is added to 100 parts by weight of Si powder having an average particle diameter of 0.1 µm, and the mixture is kneaded by a ball mill for 24 hours, to make a slurry. The slurry is subjected to vacuum degassing to remove gas bubbles, and is cast into a porous mold (plaster) to shape a 10 mm-thick pipe having a diameter of 150 mm, using a centrifugal molding machine. A composite fiber (3 mm in diameter) consisting of 50 vol percent of carbon fibers 100 µm in diameter and 50 vol percent of Al 2 O 3 fibers 100 µm in diameter is arranged on the porous mold surface in a specified pattern, and the slurry is cast around it. After the wax is removed from the molded body by heating, it is treated at 2100°C in a nitrogen atmosphere for three hours. In this way the conductive element whose resistance is measured for life diagnosis is installed in the pipe (as shown in Fig. 10). When used as a corrosion-resistant piping part, this pipe shows the change in resistance pattern of Fig. 11 before the ceramic part is damaged by corrosion. This enables prediction at any time of the remaining useful life, i.e. life diagnosis. Metallic aluminum powder may be used in place of the metallic Si powder to manufacture a AlN ceramic piping part of the same shape and the same behaviour of its conductive element. EMBODIMENT 13 To manufacture an inner lining part of a gas turbine combustor, 13 parts by weight of a mixture of polyethylene, stearic acid and synthetic wax is added to 100 parts by weight of metallic Si powder having an average particle diameter of 1 µm, and the whole mixture is kneaded. It is then made into a 10 mm-thick, 100 mm-square plate by an extruding machine. A composite long fiber (1.5 mm in diameter) comprising carbon fibers (10 µm in diameter, 1000 µm in length), Al 2 O 3 fibers (5 µm in diameter, 1000 µm in length) and SiC fibers (10 µm in diameter, 100 µm in length) is formed as a conductive element by molding on this plate. The mixing ratio of the carbon, Al 2 O 3 and SiC fibers is 30 to 50 to 20 vol percent. The molded body with the composite fiber is heated at a temperature rise rate of 1°C/min. up to 1350°C in the furnace in a nitrogen atmosphere held for 1 hour at 1350°C and cooled in the furnace to obtain a reaction sintered Si 3 N 4 ceramic provided with the composite fiber as a conductive element for life diagnosis. The binder is degreased during the temperature rise. To provide heat resistance, a compact 0.3 mm-thick ZrO 2 film is formed on the surface of the ceramic. Since the surface of the reaction sintered material is porous, this film is strongly adhered. Copper wire is then brazed on terminals of the conductive element, and connected to an external resistance measuring instrument. This ceramic part is used in the inner lining of the gas turbine combustor, with the result that the change in resistance of the conductive element shows a similar pattern to that of Fig. 6, while at the same time ZrO 2 film was damaged by the combustion in the combustion test. EMBODIMENT 14 Metallic aluminum powder is used in place of the metallic Si powder in Embodiment 13 to manufacture an A1N ceramic part having the composite fiber as a conductive element for life diagnosis, by the reaction sintering method. This part is used in the inner lining of a gas turbine combustor, and gave a result similar to that shown in Fig. 6. Various methods can be employed to form the conductive element whose resistance changes. A slurry comprising fiber as carbon fiber and a carrier medium may be cast into a mold having the desired shape of the element. Fig. 15 shows a mold 44 with a groove 43 of the desired shape. After being heated, the element 2 is taken out of the mold and is sintered integrally with the ceramic molded body 1, as shown in Fig. 15. Alternatively the slurry comprising the fiber as carbon fiber and the medium is printed into the desired pattern on the surface of the ceramic molded body, and is then sintered integrally with the molded body. The slurry may be made into the desired pattern by applying it from a nozzle 42 as shown in Fig. 14. After being heated, the element is sintered integrally with the ceramic molded body. EMBODIMENT 15 To manufacture another part for the inner lining of a gas combustor, 13 parts by weight of a mixture of polyethylene, stearic acid and synthetic wax is added to 97 parts by weight of metallic Si powder having an average particle diameter of 0.1 µm, 1 part by weight of sintering agent BeO and 2 parts by weight of AlN, and the whole mixture is kneaded. It is then made into 15 mm-thick, 100 mm-square plates by an extruding machine. Composite long fibers (2.0 mm in diameter) comprising carbon fibers (100 µm in diameter, 10 µm in length) and Al 2 O 3 fibers (100 µm in diameter, 1000 µm in length) are formed as five separate conductive elements 2 for life diagnosis (Fig. 16) by molding between two such plates. The mixing ratio between the carbon and Al 2 O 3 fibers is 60 to 40 vol percent. To detect wear depth in combustion, the depth of the elements 2 from the surface film 3 described below is varied, as shown in Fig. 16. The molded body is then heated at a temperature rise rate of 1°C/min. up to 2150°C in a furnace of argon atmosphere and is held for three hours at 2150°C. Then it is cooled in the furnace to obtain the atmospheric pressure sintered SiC ceramic body 1 provided with five conductive elements 2 for life diagnosis. The binder is degreased during the temperature rise. To provide heat resistance, a compact 10 µm-thick Al 2 O 3 surface film 3 is formed on the surface of the ceramic body by plasma spraying. Copper wire is then brazed on the terminals of the elements 2, and connected to an external resistance measuring instrument 6. This in turn is connected as illustrated to a thermal power plant diagnostic system. This ceramic part is used as illustrated in Fig. 16 in the inner lining 40 of a gas turbine combustor, with the result that patterns of change in resistance shown in Fig. 17 are observed when the wear proceeds and the Al 2 O 3 film 3 is damaged. This shows that such a ceramic part provides information about wear and the life of the inner lining of the combustor. EMBODIMENT 16 To manufacture ceramic bearing parts of the invention (Fig. 18), 13 parts by weight of a mixture of polyethylene, stearic acid and synthetic wax is added to 95 parts by weight of Si 3 N 4 powder having an average particle diameter of 1 µm, three parts by weight of Al 2 O 3 powder and 2 parts by weight of Y 2 O 3 , and this mixture is kneaded. It is then made into a inner and outer races 56, 57 of a ball bearing 50 mm in diameter in an injection molding machine. A composite long fiber (1.5 mm in diameter) comprising carbon fibers (10 µm in diameter, 1000 µm in length), Al 2 O 3 fibers (5 µm in diameter, 1000 µm in length) and SiC fibers (10 µm in diameter and 100 µm in length) is employed as conductive elements 2 for life diagnosis in the inner and outer races of this bearing. The mixing ratio of the carbon, Al 2 O 3 and SiC fibers is 50 to 30 to 20 vol percent. After shaping, each molded body is heated at a temperature rise rate of 5°C/min. up to 1750°C in an atmosphere of nitrogen, held for 1 hour at that temperature and cooled in the furnace to obtain an Si 3 N 4 ceramic body provided with the elements 2. The binder is degreased during the temperature rise. Copper wire is then brazed on the terminals of the elements 2 and connected to an external resistance measuring instrument. These ceramic bearing parts are applied to the spindle of a machine tool. Change in the resistances of the elements 2 appeared when the surface of ceramic parts contacting the bearing balls was worn and damaged in a rotational test. The invention is applicable in this manner not only to ball bearings but also to a great variety of bearings. It is also applicable for example to the mechanical seal parts of a pump which are subjected to wear, and other parts such as a slurry transport ball valve, a nozzle, a pump stator and rotor, a pump casing, and a pump shaft. Figs. 19 to 24 are enlarged diagrammatic views of various preferred forms of the conductive elements 2 whose resistance varies in the ceramic bodies of the invention. Fig. 19 shows a conductive element in the form of a bundle entirely of long conductive fibers 9, generally parallel to each other. Fig. 20 shows a conductive element consisting of a bundle of short electrically conductive fibers (whiskers) 7. The conductive element of Fig. 21 is a bundle of short electrically conductive fibers (whiskers) 7 with insulating particles 12 interspersed among them. The conductive element of Fig. 22 is a bundle of long electrically conductive fibers 9 and long insulating fibers 10, while that of Fig. 23 is formed of short electrically conductive fibers (whiskers) 7 and short insulating fibers (whiskers) 8. Finally, Fig. 24 shows an electrically conductive element made up of short conductive fibers (whiskers) 7 and short insulating fibers (whiskers) 8 having both conductive particles 11 and insulating particles interspersed among them. There may be a large number of fibers (whiskers) in the cross section of the element. EMBODIMENT 17 As illustrated in Fig. 25, means for calculating residual life in apparatus of the invention may comprise a data storage device 53 which stores specified information in advance, calculation device 52 to carry out the specified calculation according to the programs and output means such as a CRT 54 which displays the residual life given by calculation device 53. Storage device 53 comprises: (1) a first storage means 131 which stores in advance the relationship between the change of the electric resistance of the conductive element of the ceramic body and for example the amount of embrittlement representing the degree of embrittlement caused by exposure of this body to high temperature or irradiation by particles such as neutrons, (2) a second storage means 132 which stores in advance the relationship between the high temperature or irradiation of particles for the body and the amount of embrittlement, as well as the amount of use corresponding to the critical amount of embrittlement at which this specimen starts to break, and (3) a third storage means 133 which stores in advance the relationship between the high temperature or number of irradiated particles for a test piece having almost the same composition as the said body, and the time period of exposure of said test piece to the high temperature or irradiation of particles. The contents stored in storage device 53 will be explained with reference to Fig. 27. Fig. 27 (a) is a graph denoting the data in the first storage means, where the vertical axis denotes the change in resistance of the conductive element, while the horizontal axis shows the amount of embrittlement. The change in electric resistance shown by the vertical axis and the volume of embrittlement shown by the horizontal axis relate to the ratio of crack formation before and after exposure to the high temperature or irradiation of particles (i.e. breaking strength after the exposure to high temperature or irradiation of particles relative to breaking strength before such exposure). Fig. 27 (b) is a graph showing the contents in the second storage means; the vertical axis shows use at high temperature or number of particles irradiated onto the specimen, while the horizontal axis shows the amount of embrittlement. The critical amount of embrittlement shown relates to the amount of embrittlement when critical cracking occurs. Fig. 27 (c) is the graph showing the contents of the third storage means. The vertical axis represents amount of use of high temperature or irradiated amount, while the horizontal axis shows the time period of use, where the critical time period of use is defined as the time period of use corresponding to the critical use at high temperature or irradiation amount. Calculation device 53 consisting of a CPU comprises: (1) a first calculation means 121 to obtain from the first storage device the amount of embrittlement corresponding to the measured change in resistance of the test piece, (2) a second calculation means 123 to obtain from the second storage means the amount of use at high temperature or irradiated amount for the test piece corresponding to the amount of embrittlement obtained from the first calculation means, (3) a third calculation means 123 to obtain from the third storage means the calculated time period of use for the test piece, corresponding to the amount of use at high temperature or irradiated amount obtained from the second calculation means, and (4) a fourth calculation means 124 to obtain from the third storage means the critical time period of use corresponding to the critical amount stored in the second storage means, to compare this critical time period of use to the calculated time period of use, and to calculate either the difference between the two or their ratio. This method will be explained with reference to the flow chart given in Fig. 26. The first step 141 is to input the change in resistance of the test piece exposed to high temperature or irradiation by particles. The second step 142 is to obtain the amount or degree of embrittlement corresponding to the change in resistance of the test piece from the first storage means which stores a predetermined relationship between these values in material which has the same composition as that of the test piece. The third step 143 is to obtain the amount of use or amount of irradiation for the test piece corresponding to the amount of embrittlement of the test piece, from the second storage means which stores a predetermined relationship between these values. In the fourth step 144, if the amount of use or amount of irradiation of the test piece has exceeded the critical volume, this information is output to display the need for replacement of the body (step 145). If the amount of use or amount of irradiation of the test piece is below the critical value, the calculated time period of use corresponding to the amount of use or irradiation is obtained, from the third storage means which stores in advance the known relationship of these values (step 146). The final step is to obtain either the difference between the calculated time period of use and the critical time period of use or their ratio, or both of them (step 148). The above embodiments are given by way of example only and do not limit the scope of the invention.

Claims: A ceramic body (1) which in use is subjected to stress and undergoes change as a result of said stress, said ceramic body comprising a sintered ceramic material and, in contact with said material, at least one electrically conductive element (2) which has a measurable electrical resistance, which increases in dependence on said change of said body, characterised in that said ceramic body (1) undergoes gradual change with time as a result of said stress, and said element (2) is a service life and maintenance diagnosis circuit for diagnosing and estimating service life and determining maintenance requirement, said element (2) being located at a part of said ceramic body which undergoes said gradual change with time as a result of said stress, said measurable electrical resistance thereof increasing irreversibly in dependence on said gradual change, said element (2) being formed of a composite material having combinations of components selected from (a) carbon and Al 2 O 3 , (b) carbon and SiC, (c) SiC and Al 2 O 3 and (d) carbon, SiC and Al 2 O 3 . A ceramic body according to claim 1 wherein said electrically conductive element (2) consists of material having a bulk electrical resistance of not more than 1 Ω.cm. A ceramic body according to claim 1 or 2 wherein said electrically conductive element (2), as seen in cross-section, comprises a bundle of elongate members selected from fibers (9,10) and whiskers (7,8). A ceramic body according to claim 3 wherein on average, as seen in cross-section, said electrically conductive element (2) comprises at least ten said elongate members. A ceramic body according to claim 4 wherein on average, as seen in cross-section, said electrically conductive element (2) comprises a number of said elongate members in the range 100 to 10 5 . A ceramic body according to claim 3 wherein said electrically conductive element has a diameter in the range 100 µm to 1 cm, preferably 100 µm to 1 mm. A ceramic body according to any one of claims 1 to 6 wherein said electrically conductive element has a uniform resistance along its length. A ceramic body according to any one of claims 1 to 7 wherein said electrically conductive element is embedded in and co-sintered with said ceramic material. A ceramic body according to any one of claims 1 to 8 which is selected from: a) a gas turbine part selected from a moving blade, a stationary blade, a transition piece of a combustor, a heat exchanger part, a shroud part, and a duct part, b) a nuclear fission power plant part selected from a shielding material part, a heat exchanger part and a piping part, c) a nuclear fusion plant part which is installed on the side remote from the plasma of a wall of a vacuum containment vessel, d) a part of a magnetohydrodynamic power plant for generating electrical energy by movement of a high-temperature gas, said part comprising an inner wall part of a high-temperature gas exhaust duct, e) an automotive turbocharger rotor part, and f) a tundish of a metal casting machine. A ceramic body according to any one of claims 1 to 9, wherein sintered ceramic material is structurally changed by said stress by formation of cracks. A ceramic body according to any one of claims 1 to 10, wherein said components of said composite material are in a form selected from fibers and whiskers. Apparatus comprising a ceramic body according to any one of claims 1 to 11 and monitoring means therefor, said monitoring means comprising measuring means (6) for measuring said electrical resistance of said electrically conductive element (2) . Apparatus according to claim 12 wherein said monitoring means comprises judging means (52,53) for comparing an electrical resistance value measured by said measuring means (6) with at least one predetermined electrical resistance value, and providing an output dependent upon a result of the comparison. Apparatus according to claim 13 wherein said judging means (52,53) includes comparison means for comparing said value of electrical resistance measured by said measuring means with said predetermined value and providing an output signal when said measured electrical resistance exceeds a predetermined threshold value. Apparatus according to claim 14 wherein said monitoring means includes a storage means (53) for storing a predetermined pattern of change of said electrical resistance in dependence on said change of said ceramic material, and said judging means includes comparison means (52) for comparing the measured electrical resistance with said predetermined pattern. Apparatus according to claim 13 wherein said monitoring means includes means (52,53) for calculating a value relating to a remaining useful working life of said ceramic body, on the basis of said measured electrical resistance. A method of detecting change of a ceramic sintered body (1) according to claim 1 caused by stress, characterised by sensing electrical resistance of the electrically conductive element (2) intimately contacting a sintered ceramic material of said body (1) and assessing said change of said body from change of said sensed electrical resistance. A method according to claim 17 wherein said change of said ceramic sintered body (1) is change of a mechanical property thereof. A method according to claim 17 or claim 18 wherein said electrically conductive element (2), as seen in cross-section, comprises a bundle of elongate members selected from fibers (9,10) and whiskers (7,8). A method according to any one of claims 17 to 19 wherein said step of assessing said change of said body is performed by comparing said sensed electrical resistance with at least one predetermined value thereof. A method of determining a value of a remaining useful working lifetime of a ceramic body (1) according to claim 1, which body in use is subjected to stress and undergoes gradual change caused by said stress, characterised by the steps of sensing, after a period of time of use of said body, electrical resistance of the electrically conductive element (2) contacting ceramic material of said body, and calculating said value of remaining useful working lifetime in dependence upon the sensed electrical resistance. A method according to claim 21 wherein said step of calculating said value comprises comparing said sensed electrical resistance with a predetermined pattern of variation of the electrical resistance of said electrically conductive element in dependence upon said change of said body. A method according to any one of claims 17 to 22 further including the step of relieving thermal stress in said body by heating said body by electrical resistance heating of said electrically conductive element.

Inventors: KIKUCHI SHIGERU, MIYATA MOTOYUKI, SAITO YUKIO, YASUTOMI YOSHIYUKI, KIKUCHI, SHIGERU, MIYATA, MOTOYUKI, SAITO, YUKIO, YASUTOMI, YOSHIYUKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: G01N33
IPC codes: G01L1, G01N33, G01N27