Organic Inorganic Conversion Research Articles

Effect of the polymerization degree of photopolymers on the thermal and mechanical properties of ceramic cores

A ceramic core must have a high fracture strength before and after heat treatment to reasonably handle the complex and thin parts. To develop such a ceramic core, herein, a photocurable monomer with high network density was applied as an organic binder to increase green strength, and an inorganic binder was additionally introduced to minimize the shape distortion of the core due to the organic–inorganic conversion process. Fused silica beads as a starting material and trifunctional and monofunctional acrylate as an organic binder were used; these slurries with various compositions formed the green body of the core shaped through a pressing process. Next, the green body was subjected to UV irradiation for 10 min to photopolymerize the monomer, followed by heat treatment at 900 °C for 1 h to convert the inorganic binder into a glass phase and to decompose the polymer. The as-prepared ceramic core exhibited a high strength of ∼20 MPa before heat treatment owing to the presence of the crosslinked polymer with high network density. By converting the inorganic binder into a glass phase, it was possible to manufacture the ceramic core at a strength of ∼40 MPa. The photopolymerization behavior of monofunctional and trifunctional acrylate monomers and a mixture of both monomers was analyzed by FT-IR spectroscopy, and the thermal behavior of each ceramic core sample was analyzed by TG-DTA. The relationship between the polymerization degree of the photocurable monomer and strength was investigated via thermal analysis and microstructure measurement.

Improvement in the hygroscopicity of inorganic binder through a dual coating process

The use of an anti-absorbent is proposed in this work to reduce the hygroscopicity of the inorganic binder in the casting mold, in which the anti-absorbent is coated on the mold prepared with an inorganic binder. Three types of polymers were used to select material with optimal water resistance. Polystyrene (PS) and polyvinyl alcohol (PVA) were used as a water-insoluble polymer and water-soluble polymer, respectively. In addition, polyurethane (PU) prepolymer has intermediate properties between PS and PVA. PVA and PU prepolymer were used for comparative testing with PS. For this testing process, the prepared green body was dipped into a solution of inorganic binder precursor mixed with tetraethyl orthosilicate (TEOS, SiO2 precursor) and sodium methoxide (NaOMe, Na2O precursor), and then dipped into a solution of coating reagent after a drying process. Thus, these series of coating processes in a green body is called a dual coating process. Finally the sample was heat-treated at 1000 °C to generate a glass phase by an organic–inorganic conversion process. In the sample prepared with PS, the highest contact angle and a high firing strength were exhibited, independent of polymer concentration, while the sample coated with PVA showed lower green and firing strengths. When prepolymer, PU, was applied, the green strength was remarkably improved, showing lower firing strength compared with that of PS. The green and firing strengths were optimized through the dual coating process with PS. Moreover, the moisture-proof effect of the dual coating process was verified through the moisture steam test.

Process development of fabricating ceramic core using 3D printing technique

Previous 3D printers developed for the fabrication of ceramic cores have been limited to implement mechanical properties and provide shapes that meet the requirements, since the type and size of powder, and the type of binder are standardized according to the printer. Therefore, in this study, the composition and manufacturing process of ceramic powder and binder were studied to fabricate ceramic cores with high mechanical properties without depending on the type of 3D printer. Fine mullite powder (average particle size: 16 μm) and zircon flour (average particle size: 43 μm) with conventional mullite bead. In addition, two types of polyvinyl alcohol (PVA) were used to enhance the coating ratio of an inorganic binder. Green bodies were formed by a three-dimensional (3D) printer employing three kinds of starting powders and PVAs mixed at the proper ratio. The green bodies were then heat-treated at 250 °C to evaporate the PVA with a lower boiling point. Then, the heat-treated core samples were dipped into the inorganic precursor, dried, and heat-treated at 1000 °C for the organic–inorganic conversion process. Through using a combination of different starting powders, the compact density of the sample was increased and the pore size was reduced, resulting in an increase in the coating efficiency of the inorganic binder. This study demonstrates the feasibility of the fabrication of a ceramic core with excellent strength through the 3D-printing process.

Application of dual coating process and 3D printing technology in sand mold fabrication

A new dual coating process has been developed for complex sand mold casting using three-dimensional (3D) printing technique. In the dual coating process, two types of polyvinyl alcohols (PVAs) with different decomposition points were applied, unlike conventional casting methods that use just one organic binder. Mullite bead (~200μm), mullite powder (~16μm), and zircon flour (~43μm) were used as starting materials, which were coated with PVA aqueous solution and then recoated with an inorganic binder. The coated powders were heat-treated at 1000°C for 1h, followed by a 3D printing process. The sample was heat-treated at 250°C for 4h to burn out the PVA with a lower decomposition point. The heat-treated sample was dipped into the solution of the inorganic binder composed of tetraethyl orthosilicate and sodium methoxide, which are the silica and sodium oxide precursors, respectively. After drying at 80°C for 1h, the final heat treatment was conducted at 1000°C for 1h to produce an organic–inorganic conversion reaction. In the case of the starting materials composed with the same composition, as the particle size became smaller, higher surface roughness and strength were obtained. The highest green and firing strength could be obtained in the samples with zircon flour and mullite bead, respectively. These results confirm the feasibility of combining a new coating process with 3D printing technology.

New conversion process for fabricating a ceramic core by a 3D printing technique

A new conversion process combined with a three-dimensional (3D) printing technique has been developed for application in the fabrication of a ceramic core. The new process allows the provision of a sufficient amount of inorganic binder on the particle surface and at the interface between particles. Two types of polyvinyl alcohol (PVA) with a similar molecular structure, but with a large difference in decomposition temperature were used as an organic binder. A green body was prepared with two kinds of PVA using a 3D printer, which was heat-treated at 250°C to evaporate the PVA with the lower decomposition point. The heat-treated core samples were dipped into the inorganic precursor, and then dried and heat-treated at 1000°C for the organic–inorganic conversion process. In the new conversion process, the formability of the core sample was controlled through the remaining organic compounds and the inorganic binder penetrating between particles and/or coated on the particle surface, and the firing strength was attributed to the glass phase generated by the inorganic precursor. The firing strength of the core prepared through the new process was improved because of the increase in the inorganic precursor that infiltrated the spaces/sites of the decomposed PVA, and it could be readily applied to the production of a core using 3D printing techniques without further shrinkage in a high-temperature heat treatment.

Organic-Inorganic Conversion Process for Material Creation -Formation and Function of Characteristic Nanostructures-

In the case that an organometallic compound or an organic metal complex is used as the raw material in an “organic–inorganic conversion” process, the desired shape and characteristic nanostructure can be formed. This nanostructure can provide unique functionality and lead to the creation of original materials. From a fibrous form of polycarbosilane, which is an organosilicon polymer, made by melt spinning in an inert gas, continuous inorganic SiC fibers can be obtained. These fibers are not easily oxidized and their tensile strength does not decrease even in a high temperature in air. They can therefore be used in fiber-reinforced composite materials. Polycarbosilane is also an excellent binder for ceramic powder that is difficult to sinter. In addition, it is an excellent impregnation agent for ceramic compacts. Thus, preparation of ceramic compacts having high mechanical strength and oxidation resistance is possible using polycarbosilane. On the other hand, when bis(acetylacetonato)zinc; (Zn(acac)2), which is an organic metal complex, is made into a fibrous form by sublimation and pyrolysis with superheated steam, inorganic ZnO fibers can be obtained. Such fibers exhibit visible-light photocatalytic ability and effectively decompose volatile organic gases. [DOI: 10.1380/ejssnt.2011.181]

Synthesis of a minute SiC product from polyvinylsilane with radiation curing: Part II—Ceramization process of radiation cured polyvinylsilane

We have synthesized minute SiC products from polyvinylsilane (PVS), which is a liquid organosilicon polymer, with radiation curing. Since there is a close relationship between the properties of obtained SiC products and pyrolysis condition, it is important to investigate the ceramization process of cured PVS in order to find out the optimum pyrolysis condition. In this paper, the ceramization process of the PVS cured by γ-ray irradiation at room temperature was investigated by gas analysis, thermogravimetric analysis (TGA), density measurement, and so on. It was found that the ceramization of γ-ray cured PVS starts above 500 K, and drastic organic–inorganic conversion occurs in the temperature range of 700–1100 K. According to the changes of mass and density, it was found that the volume shrinkage of PVS during the curing and pyrolysis processes is 80%. The SiC obtained by pyrolysis at 1573 K in argon gas atmosphere showed the density of 2500 kg/m3 and microvickers hardness of 31.6 GPa.

Characterization of precursor-ceramic conversion in Si-C-(O,N)-systems by thermoanalytical methods

On-line coupling of thermogravimetry (TG) with evolved gas analysis (EGA)/mass spectrometry (MS) was applied to characterize the organic-inorganic conversion of two synthesized polysilazanes to Si-C-N materials and of one commercial polysiloxane to Si-C-O materials.