Process for manufacturing carbon composites saturated with molten metal such as antimony using a plasma spark sintering machine

Abstract

This invention relates to the field of the process of manufacturing carbon composites saturated with molten metals for use in mechanical seals and the manufacture of carbon / ceramic composites. Graphite impregnated with molten metals and alloys such as antimony or silver can be used in the manufacture of mechanical seals depending on the operating temperature and the desired fluid. The purpose of this invention is to use the SPS method to produce this type of composite and to introduce this method as a process for impregnating porous carbon materials with other molten materials. In this way, by using mechanical pressure and high temperature of the melt inside the mold and with the help of vacuum outside the mold using a spark plasma sintering device, graphite is saturated with molten antimony. Using this method, it is also possible to make various types of graphites saturated with other metals. For example, the depth of antimony inoculation was a remarkable 3 cm in 9 min. The density, porosity percentage, and flexural strength of graphite before antimony inoculation were 1.68 gr / cm3, 21.1%, and 21 MPa, and after antimony inoculation were 2.12 gr / cm3, 14%, and 33 MPa, respectively.

Description

Description of the invention Title of the invention The process of manufacturing carbon composites saturated with molten metal such as antimony using a plasma spark sintering machine Technical background of the relevant invention This invention relates to the field of the process of manufacturing carbon composites saturated with molten metals for use in mechanical seals and the manufacture of carbon / ceramic composites. Technical problem and stating the objectives of the invention Carbon materials, including graphite bodies and carbon / carbon composites, have attracted attention due to their special properties. Most of the applications of these materials are due to their suitable properties at high temperatures. These materials have high strength and maintain their strength even up to temperatures above 2000 oC and in a neutral environment. One of the applications of carbon materials is the use in mechanical seals. Mechanical seals are a necessary tool in pumps used in petrochemical, power plant, chemical industries, etc. Mechanical seals mainly consist of a rotating seal surface with a rotating mechanism that rotates at the same speed as the pump shaft and a fixed seal surface that is connected to the rotating seal surface. Graphite impregnated with molten some metals and alloys such as antimony, babbitt, silver, etc. has wide applications in the manufacture of mechanical seals depending on the application temperature and the desired fluid. For example, graphite mechanical seals impregnated with antimony are mainly used for fluids with low viscosity.These types of graphite composites saturated with molten metals are imported and are required by various industries, and the technical knowledge of their standard production is not available. These types of composites are expensive due to their high production technology, and achieving a method with high efficiency and depth of saturation is of particular importance. The purpose of this invention is to use a new method to produce these types of composites and even similar composites for other applications. In this way, by using high mechanical pressure of the melt inside the mold and vacuum of the chamber using a spark plasma sintering device, the saturable graphite is explained by molten antimony. It should be noted that the aforementioned innovative method has not been reported in the references and its related parameters are unknown, and the purpose of this invention is to prove the efficiency of this method in producing this category of materials. It is expected that using this method, a suitable efficiency for deep penetration of antimony into graphite will be achieved, and this process can also be used to manufacture various types of graphites saturated with other metals. A description of the state of the prior art and the history of developments related to the claimed invention. In an article titled ''Materials for Mechanical Seals'' published by Springer Science in 2013, carbons used for sealing were discussed from pages 2173 to 2183. The following is a brief summary of the highlights of this work. This material is the basis of organic processes and products and is of great interest given that it exists in various species including amorphous carbon, graphite, diamond and fullerene. Carbon is an inert and stable material and has self-lubricating properties. This material is used in a variety of applications including pigments, carbon black in rubber and electrical connectors and can be used in the form of soft graphite powder or hard friction plates. Mechanical carbons used for sealing are a mixture of amorphous carbon and graphite. The percentage of each of these components determines its physical properties and grade. In addition to carbon, other elements and compounds are used in different grades and determine the properties of the grade used. Some of these are impurities resulting from the original source of carbon and others are used as additives to improve performance.These additives are used to control the formation of the intermediate film and include silicon oxide, silicon carbide, molybdenum disulfide, or lithium fluoride. Mechanical carbons are made by mixing amorphous carbon (e.g., carbon black, charcoal, or coke) and graphitic carbon with a carbon binder such as bitumen or resin. The source of the raw materials determines the physical properties and the type and amount of impurities. The use of other additives in the mixing operation depends on the desired grade. The mixture is then pressed into the desired shape and heated in an inert environment. At high temperatures, the binder decomposes into carbon and gaseous substances and is expelled from the part, resulting in the creation of porous, soft carbon. The carbon is then placed in a vacuum chamber to remove air from the pores, and a liquid impregnation process is carried out under vacuum. The chamber is then pressurized to fill the pores, and effective filling of the pores leads to the impermeability of the part to fluids and also increases the strength of the final product. The choice of filler type is a very important factor in determining the final properties of the product.The filler is made of various materials such as thermosetting and thermosetting resins, various metals and salts. The most common and popular fillers for use in sealing surfaces include thermosetting resins and antimony. While carbon manufacturers produce hundreds of different grades of carbon, 12 grades have become standardized in the mechanical seal industry for various applications. Arunachalama and his colleagues studied the penetration of molten metal into the initial preform for the manufacture of composites (infiltration) in an article published in 2019 in the Journal of Manufacturing Processes, Volume 42 (pages 213-245). This process can be carried out without pressure or with pressure. In infiltration, the molten reinforcing phase is first placed in the mold and the molten alloy penetrates it and then solidifies. This process can be carried out with the help of an external force through mechanical pressure, gas pressure, vacuum, steam, centrifugal force, etc. Chen and colleagues conducted research on graphite impregnated with phenolic resin in 2021, and their paper was published in Earth and Environmental Science, Vol. 692 (pp. 1-6). Graphite alone had excellent thermal stability, thermal conductivity, and corrosion resistance, and phenolic resin as a filler for graphite pores often had high density and low working temperature, and its thermal expansion coefficient was 10 times that of graphite. Phenolic-impregnated graphite was suggested for highly corrosive environments such as acidic environments. Stec et al. have inoculated molten iron into a carbon refractory with micron-sized pores. The process used is a 3 bar gas pressure furnace and a molybdenum disilicide heat source at a temperature of 1500°C and a time of 90 min for the penetration of molten iron into the porous carbon. Ma et al. fabricated a carbon / aluminum composite with a compressive strength of 304 MPa using the mechanical stress penetration method. The mechanical pressure was 3 MPa and the temperature was 850°C for 1 h. The reason for the high strength is the formation of an aluminum carbide phase at the interface of the composite components. Li et al. first made a carbon porous body by hot pressing a mixture of petroleum coke, graphite and carbon black at 150°C and then carbonization at 1150°C, and then penetrated molten copper into the carbon porous body by mechanical pressure. The density of the initial carbon body was 1.366 gr / cm, the density of the resulting composite was 2.352 gr / cm, and its flexural strength was 47.52 MPa. Among the materials used to fill the pores of carbon / graphite, antimony has a special place. The working temperature and strength of graphite / antimony are higher than those of resin-impregnated carbon. Antimony is also cheaper than other metals such as silver and copper. Graphite / antimony has recently been used in mechanical applications, but little scientific research has been published in this field. Wang et al. fabricated a porous graphite body using coal tar pitch and then infiltrated it with antimony using pressure inoculation. To make a graphite / antimony composite, coal tar pitch is first converted into a fine-grained powder with different grain sizes and proper distribution. In order to achieve carbon bodies with optimal porosity distribution, the following granulation formulas were proposed: 200-400 mesh (45%), 600-400 mesh (25%), 800-600 mesh (15%), 1000-800 mesh (10%), and 2000-1600 mesh (5%). High temperature coal tar pitch was used as a binder and was added to the initial granulation at a rate of 30% by weight and mixed for 4 h. Then, this mixture was heated at 170 oC for 1 h.Then, the graphite raw body was made by molding at a pressure of 70 MPa and a time of 5 min and finally it was carbonized at a temperature of 1000 oC with a special temperature program. In order to inoculate antimony, the carbon body made was stabilized in a crucible so that the molten material would not float during penetration. Then, antimony was placed in the crucible and the temperature was increased to 800 oC (melting point of antimony oC 631). Then, this crucible was placed in a penetration chamber and the chamber was evacuated and then subjected to a mechanical pressure of 12 MPa for 5 min and after cooling and removal, the desired composite part was made. Providing a solution to an existing technical problem along with an accurate, sufficient, and integrated description of the invention The purpose of this invention is to introduce a new method for inoculating molten metal such as antimony into a graphite body and to explain the microstructure and mechanical properties of the resulting composite. It seems that the mechanical pressure inside the mold combined with the suction caused by the vacuum of the SPS chamber has a significant effect on the penetration of the molten material into the dense graphite, and the purpose of this invention is to prove the feasibility of this process for inoculating molten material into a porous body (even with micron porosity). According to the research of the inventors of this invention, this method can also be used for inoculating molten materials of other metals at different temperatures. Here, a metal such as antimony with a low melting point was used to demonstrate the feasibility of this method. 1-Raw materials The materials used include carbon (graphite) and antimony powder. This piece is made by grinding the initial carbon and graphite precursors and then pressing and carbonizing at 1050 oC without bitumen inoculation. The density of the carbon piece used was 1.368 gr / cm and the apparent porosity was 21.1%. Also, the antimony powder used was industrial antimony powder with a purity of 99% and a mesh size of 325. 2-Equipment and tests used For phase analysis, a Philips pw 3710 XRD device with CuKα radiation and a wavelength of 1.54 Å was used, and finally the relevant phases were identified using High score Xpert software. For microstructural analyses, a TESCAN VEGAII electron microscope equipped with a Rontec EDS spectrometer was used. Optical microscope images were also obtained and analyzed using Imagej software. To determine the flexural strength of the samples, a SANTAM model 20T device was used. The length and width of the samples were 6.4 × 5.0 mm, the distance between the two supports was 30 mm, and the load application speed was 0.21 mm / s. Figure 1 shows the schematic of the SPS device. The device used was manufactured by EasyFashion Metal Products Co., with a maximum power and compressive force of 100 KW and 20 Ton and was manufactured in China. The device consists of various units including a power supply, a vacuum chamber, a control panel, a cooling unit, a vacuum pump, and a hydraulic pump to provide pressure. The temperature was measured by a thermocouple from the sample surface area. For non-destructive analysis of the amount and geometry of antimony penetration in graphite, the radiographic test of Pars Precision Casting Company was used. First, the sample was cut in half after antimony inoculation and X-ray test was performed from three directions as shown in Figure 2. This test is performed using common radiographic films and using X-ray and gamma ray devices, taking into account the thickness and geometric complexity of the piece so that the entire piece is covered. The basis of the radiographic test is based on the difference in absorption of radiated energy inside the object. In this method, high-energy rays such as X-ray and gamma rays are used to penetrate the object. By placing the film under the piece, the rays sent after passing through the object leave their mark on the film and by observing the films, possible defects in the piece such as porosity, impurity confinement, cracks, etc. can be identified. The films are developed in a place called a darkroom by an X-Ray film processor and finally the resulting images are interpreted. Figure 3 shows a schematic of the dimensions and components required for inoculating molten antimony into graphite. These components were made by machining graphite to the desired dimensions. Graphite foil was used to ensure that molten antimony did not penetrate or to minimize it in the mandrel. Figure 4 shows the image of the manufactured components. After the mold was made, all components were weighed and 51.5 gr of antimony powder was poured into the mold cavity and three layers of 1 mm thick graphite foil were placed on it. Then the mandrel was placed in its place and the entire assembly was placed in the SPS chamber and between the two jaws of the machine. Due to the large volume of powder and the low strength of the mold, the minimum pressure of the machine was used, namely 0.5 Ton (10 Mpa). Then the machine flow was applied at a speed of 0.04 A / min and data related to the temperature and displacement of the machine jaws were recorded. Figure 5 shows a schematic of this process. Then, according to the displacement of the two jaws, the time of complete discharge of the melt into the graphite pores was detected, and as soon as the melt began to discharge, the temperature stabilized, and after the melt was completely discharged, the flow rate decreased and the sample was slowly cooled. Then, the sample was removed from the chamber of the device and cut in the middle. So that two parts of solid and hollow cylinders were obtained. According to Figure 2, X-ray images were obtained from both parts and from three different viewing angles, and then samples were cut from the penetrated part to determine the density, microstructure, and bending strength. Explanation of shapes, maps and diagrams Figure 6 shows an X-ray image from the top view of the samples (Y axis in Figure 2). Radiographic testing refers to the use of gamma and X-rays, which can penetrate many materials, to analyze materials and detect product defects. In this method, X-rays or radioactive rays are directed towards the part and are reflected on the film after passing through the part. The thickness and internal characteristics cause spots in the film to appear darker or brighter. X-rays and gamma rays have very short wavelengths and therefore have a lot of energy and have the power to penetrate and pass through the part. The passage of these rays through any environment is accompanied by attenuation and absorption of part of it by the environment. The amount of attenuation is affected by several factors, including the density and structure of the environment, as well as the type, intensity and energy of the ray. The basis of this method is the change in the absorption coefficient and the change in the amount of radiation passing through the healthy and defective parts of the part. The presence of any defect that has a different density than the part causes a decrease or increase in the amount of radiation passing through the part.Using X-ray film, these rays are recorded and after the film is developed, the film defects can be interpreted (Figure 7). After development, the X-ray film turns black due to the radiation, and the areas that received more radiation will be darker and the areas that received less radiation will be lighter. Defects such as oxide grains that have a higher density than the part have a higher absorption coefficient and reduce the intensity of the transmitted radiation, as a result of which these points leave a brighter effect on the film, or vice versa, defects such as holes and gas bubbles that have a lower density leave a darker effect on the film. By carefully interpreting the film and familiarizing yourself with the process performed on the part, it is possible to comment on the defects or possible content inside the part. The amount of radiation absorption is directly proportional to the mass of the material. The amount of mass depends on the density or chemical composition of the part and equally on the amount or thickness of the part. In general, the higher the atomic number of the material, the more radiation is absorbed and the less radiation penetrates the part to reach the film. On the other hand, the amount of radiation that passes through thin sections is greater than thick sections. Dark areas of the image indicate parts of the test piece that penetrated more easily because more radiation has passed through it and had a greater effect on the film. Whereas lighter areas indicate areas of the piece that are more difficult to penetrate.Therefore, using this method, it is possible to know with high confidence the geometry of antimony penetration into the part. Considering that the fabricated sample contains only carbon and antimony, the black area can be attributed to porous carbon with an atomic mass of 12 gr / mol and a theoretical density of 2.32 gr / cm, and the white area is related to the presence of antimony with an atomic mass of 121.7 gr / mol and a density of 6.37 gr / cm in the carbon pores. According to Figure 6, preferential penetration of antimony into graphite is observed, which is due to the anisotropic structure of the initial graphite block. Because the initial block was made using the bidirectional pressing method, and the preferential orientation of the grains and, as a result, the preferential orientation of the pores caused the non-uniformity of the melt penetration. This effect can also be attributed to the non-uniform distribution of the flow in the mold and, as a result, the non-uniform distribution of the temperature in the mold due to the anisotropic properties of the initial block. So that in the areas with higher flow, higher temperatures were achieved, leading to a decrease in the viscosity of the melt and the tendency of the melt to move towards areas with higher temperatures. Considering the observation of capillary paths in the advancing front, the inoculation of the antimony melt with the help of capillary force is evident in the graphite pores. The electrical resistance of antimony is about 7-10 × 4 mΩ and the electrical resistance of graphite is in the range of 6-10 × 5 to 6-10 × 30 mΩ. The lower resistance of antimony leads to greater current flow and higher local temperatures, resulting in rapid melting of antimony.The dark circle in the top view X-ray image indicates the successful performance of the graphite foils in preventing the penetration of antimony into the mandrel and its complete evacuation into the graphite cylinder. The reason for this phenomenon can be attributed to the lack of suitable porosity in the graphite foil for the passage of the melt towards the mandrel. Figure 8 shows an X-ray image from a viewing angle around the cylinder (X and Z axes in Figure 2). Several conclusions can be drawn from this image. First, the amount of melt penetration towards the bottom of the cylinder is greater, which could be due to the availability of more melt and also to the force of gravity. Second, the geometry of the antimony-containing space formed in the middle of the mold is an elliptical sphere, and as explained, the reason for this geometry is probably the anisotropy of the initial graphite block. Third, the maximum depth of antimony penetration was about 30 mm, which is a significant number. The reason for this phenomenon is the emptying of the molten reservoir due to the mechanical pressure of the mandrel into the cavities and the suction of the external vacuum. It seems that the inoculation process is stopped when the antimony is exhausted, and if molten antimony was still available and present in the reservoir, the penetration depth would have increased. After removing the mandrel from the graphite mold, there is no trace of antimony in the mold cavity. Therefore, the antimony powder has melted and penetrated into the graphite or has exited the mold through the seam between the mandrel and the matrix. Evidence does not show any trace of the melt exiting the mold, but the formation of a white layer on the inner wall of the SPS chamber indicates the formation of antimony vapor. By comparing the weight of the mold and powder assembly before and after the process, it was concluded that most of the antimony powder has melted and penetrated into the graphite, and of the total 51.5 gr of antimony powder, only 2.7 gr has evaporated and exited the assembly. The exit of antimony vapor is possible through the pores leading to the graphite and also through the seam between the mold and mandrel. Figure 8 shows the image of the graphite cylinder after cutting through the middle of the cylinder. In appearance, the polished surfaces show two separate parts, which indicate the penetration of molten antimony into the graphite. Figure 10 shows the temperature and displacement versus time graph for the SPS process for antimony inoculation. The melting point of antimony is 630°C. The thermocouple shows the temperature of the cylinder surface, and due to the large distance to the center of the mold (antimony powder) and the major flow through antimony, the temperature of the powder is much higher than the mold surface. The lower resistance of antimony leads to greater flow and higher local temperature, resulting in rapid melting of antimony. The displacement rate of the mandrel is directly affected by the behavior of the powder inside the mold, and due to the high height of the powder, the effect of thermal expansion of graphite during mold heating can be ignored. According to the graph in Figure 10, in area 1, antimony is sintering, and after 26 min and at a mold surface temperature of 250°C, antimony sintering is complete and reaches 100% density. Calculations of the amount of powder and antimony density also confirm this. Due to the reasons mentioned, at this moment the actual temperature of the center of the mold is much higher than 250 oC. In zone 2, the phenomenon of antimony melting and simultaneous discharge of antimony melt into the graphite pores occurs within 9 min.The temperature of the mold surface at the moment of antimony melting is 380 oC and if we ignore the spark phenomenon between particles, at this moment the temperature of the mold center is about 630 oC. With complete inoculation of the melt, the mandrel reaches the end of the mold and the movement of the mandrel stops. Therefore, with a quick and short process, the entire melt is inoculated into the graphite pores with the help of mechanical pressure and simultaneous vacuum. Figure 11 shows the phase composition of the graphite / antimony composite. In the phase composition of this composite, only the graphite and antimony phases are visible, and no trace of the formation of a compound between these two materials is observed. Also, considering the width of the main graphite peaks, the presence of amorphous carbon in the composite is evident. Figure 12 shows the backscattered and secondary electron SEM image with a magnification of 100 times of the graphite / antimony composite. Also, Figure 13 shows the SEM image with a magnification of 400 times and an elemental map of the graphite / antimony composite. The secondary electron image is suitable for detecting the surface morphology and the degree of filling of the pores. The white phase indicates the presence of antimony and the gray phase indicates the presence of carbon. Also, no trace of the formation of a compound between these two materials is observed. Most of the graphite pores are filled with antimony, but some pores are still empty of antimony, which could be due to the lack of access of the melt to these pores. Figure 14 shows the SEM image with the lowest possible magnification of the inoculated graphite sample.The white dots indicate the presence of antimony. As can be seen, the antimony has penetrated in a specific direction and continued its path. As mentioned, the reason for this orientation is due to the anisotropic structure of the initial graphite block resulting from the axial press forming method. Figure 15 shows the optical microscope image of the initial graphite at 50 and 200 times magnification. The porous structure of graphite is due to the manufacturing method and the release of gases from the pyrolysis and carbonization of the raw materials. The results of the Archimedes test indicate a density of 1.368 gr / cm and an open porosity of 21.1% for the initial graphite block. The flexural strength of this material has also been measured to be 21 MPa. Figure 16 shows the optical microscope image of the resulting graphite / antimony composite at 50, 200, 500 and 1000 times magnification. In the microstructure of the composite, the coarse pores of graphite are not filled by antimony. While the finer pores are well filled. So that even very fine pores with a diameter of 1 µm are well filled by antimony. The results of the Archimedes test indicate a density of 2.12 gr / cm3 and an open porosity percentage of 14% for the resulting composite. Also, the flexural strength of this composite has been measured to be 33 MPa. Optical microscope images were used to analyze the porosity and also the amount of antimony inoculated. For greater accuracy, it is necessary to select images with appropriate magnification so that the microstructural details are representative of the entire composite. Observation of the images shows that 50x magnification is appropriate for image analysis. Figure 17 shows the porosity analysis of the initial graphite before inoculation with antimony. The results of the image analysis show a porosity of about 19.5% for the initial graphite. Figure 18 shows the porosity analysis for the graphite / antimony composite. As can be seen, about 9.8% of the pores are free of antimony and include larger pores. Figure 19 shows the antimony content analysis for the graphite / antimony composite. As can be seen, about 8.5% of the microstructure is inoculated with antimony and includes small pores that are saturated. (Porosities with an area of ​​less than 22500 µm) The reason why the large pores were not successfully saturated is not completely clear, but the reason for this phenomenon can be attributed to the stronger capillary force of the small pores, which causes the antimony melt to penetrate these pores and freeze in place with the decrease in temperature. So that even pores with a diameter of 1 µm were successfully saturated by the antimony melt. However, the antimony melt was unable to saturate pores with a diameter greater than 50 µm. A clear and precise statement of the advantages of the claimed invention over prior inventions. 1- In this work, for the first time, using the SPS vacuum hot pressing method, molten metal such as antimony was infiltrated into the cylindrical pores of graphite to a maximum depth of 3 cm to make a graphite composite. Using this method, other molten metals can be inoculated into graphite. 2- Using X-ray imaging, clear and reliable results were obtained regarding the geometry and depth of penetration of molten metals into graphite. 3- In this process, using temperature data and SPS jaw displacement, optimal conditions were achieved to achieve the best melt inoculation efficiency into graphite pores without the need to manufacture multiple samples and analyze various variables. 4- The main factor in achieving high penetration depth in a short time in this process can be attributed to the mechanical pressure of 10 MPa (100 Bar) on the melt and in the opposite direction of the suction caused by the vacuum of the SPS chamber and the significant contribution to the capillary suction of the melt. In this invention, the antimony inoculation depth was about 3 cm in 9 min. 5- The density, porosity percentage and flexural strength of graphite before inoculation with antimony were 1.368 gr / cm, 21.1%, 21 MPa, and after inoculation with antimony were 2.121 gr / cm, 14%, 33 MPa, respectively. Description of at least one implementation method for implementing the invention One of the implementation methods for applying the present invention is to use this composite to make mechanical seals. Mechanical seals are a required tool in pumps used in petrochemical, power plant, chemical industries, etc. Mechanical seals mainly consist of a rotating sealing surface with a rotating mechanism that rotates at the same speed as the pump shaft and a fixed sealing surface that is connected to the rotating sealing surface. Another application of this invention is to make carbon / ceramic composites such as CC / ZrC-SiC for use at very high temperatures such as solid fuel rocket nozzles. Explicit mention of the industrial application of the invention The composite has a wide range of applications in the manufacture of mechanical seals depending on the application temperature and the fluid. For example, graphite mechanical seals impregnated with antimony are mainly suitable for low viscosity fluids. Another application of this invention is the manufacture of carbon / ceramic composites such as CC / ZrC-SiC for use at very high temperatures such as solid fuel rocket nozzles.

Claims

Claim What is claimed: Claim 1) What is claimed is a process in which molten antimony is infiltrated into cylindrical graphite pores to a depth of 3 cm using the SPS (Spark Plasma Sintering) technique to produce a graphite composite with antimony reinforcement. Claim 2) According to claim 1, antimony inoculation is carried out for 9 min under a pressure of 10 MPa and the vacuum of the SPS chamber.

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