Synthesis of group IV B zirconium and group V B niobium diborides via CVD process as free particles and evaluation of sintering properties

In this thesis project, for the first time in the world, Group IV B Zirconium and Group V B Niobium Diboride were synthesized as loose particles by chemical vapor deposition (CVD) technique. These materials are typically produced only as refractor coatings, on a micrometer scale. Powders in this project were produced to be the raw materials for bulk sintering products. Sintering tests were also carried out. Chemical vapor deposition (CVD) technique allows synthesis with cheap raw materials and permits the production of high purity, cutting edge ceramic materials. In CVD, the only factor affecting the purity of the end product is the inlet gas purity. CVD was chosen to be the production technique for this project, because it allows the production of fine grained structures. In literature, all of the CVD synthesis of Zirconium Diboride and Niobium Diboride are intended for coating applications. In this thesis project, for the first time, boride was produced as free particles via CVD. The unique process and system designs were developed by our team. Among halides, chlorides were chosen to be the raw materials since they are suitable for industrial scale production. Chlorides are an economic option because they are easy to procure, intermediate products. Processing was designed to comply with the chosen raw materials. The thermodynamic reaction characteristics of the chloride materials were investigated with the help of the modellings done with Fact-Sage. Reaction mechanisms for the ZrCl4/BCl3/H2 and NbCl5/BCl3/H2 systems were determined with the modellings done by Gibbs energy minimization. According to these reaction mechanisms, suitable gas flow rate and reaction temperatures were determined. During metal boride synthesis, BCl3, H2 and Ar gasses and ZrCl4/NbCl5 salts were used as raw materials. In the synthesis process, the CVD reactor, which was designed and manufactured by our team (Patent No: EP2735544-A2 and TR013720) and whose reaction tube is made up of quartz, was modified to be suitable for boride production. In order to maintain the product stoichiometry, gasses should be fed into the reaction environment in specific flow rates and in constant proportions. The major difficulty in achieving this is the fact that chloride salts are required to be gasified in stable flow rates. In order to accomplish this, firstly sliding hot zoned units were designed. However, desired physical efficiency was not achieved. Thus the material and position of the salt chambers were altered and vibratory feed unit was developed in order to maintain constant flow rates. Certain amount of salt was fed into the gas inlet for a certain amount of time, and was evaporated via blowpipes. With this method, gasified metal chloride salts were successfully fed into the system in controllable, stable flow rates. Firstly, boride synthesis experiments were carried out by using a quartz base. However, the produced solid phase was observed to be buried into quartz and thus the experiments were not successful. A comprehensive literature survey had shown that the nucleation of borides on a quartz surface, in free particle form, is very difficult. The process has a kinetic obstacle and requires temperatures exceeding 1300 °C. So alternative CVD base materials were explored, which would be used instead of quartz. Synthesis of the diborides mentioned above, as free particles, is not possible. However, the coatings deposited on semi-noble and noble metal surfaces, which have high surface reactivity, can be separated from their bases and thus be transformed into loose particles. This idea was worth experimenting on, and it was successfully executed by using copper as the substrate material. In the CVD reactor, borides were synthesized on copper base and by carrying out leaching with nitric acid; the substrate was separated from the coating layer. On these boride powders, phase analysis was done with XRD; microstructural analysis was done with SEM; chemical analysis was done with EDS; density measurement was done with Helium Gas Picnometer and particle size analysis was done with Zeta-Sizer. With the help of AAS, B and Zr analysis was done on the copper solution obtained from leaching. ZrB2 was coated on copper base, in varying temperatures (850-950 °C), for varying times (1-3 hours and varying reactor sizes (1.5-3 meters). After leaching with nitric acid, free boride particles were obtained. The effects of temperature, substrate thickness (0.05-0.2 mm), reactor size (retention time in the reaction environment) and reaction duration, on the efficiency of the process, were evaluated. By keeping the gas flow rate constant, and by varying the other parameters, numerous experiments were carried out. According to the results from these experiments, temperature of 950 °C, reactor size of 3 m and reaction time of 3 hours yielded the highest efficiency. With these parameters, gas phase to solid phase transformation efficiency was 28%. In the SEM images of the copper folio, which was coated in the experiment executed at 900 °C, the primary wetting film formed on the base metal was observed. It was observed that, the morphology of the coatings obtained from the experiments done at 900 °C and 950 °C, were very different. At 900° C, bulky and spherical structure was observed, whereas at higher temperatures (even with a 50° C increase), dendritic structure was observed. In the beginning, Zirconium Diboride was deposited layer by layer. When its temperature was increased, the excess energy was absorbed as surface energy, thus dendrites were formed. Following the experiment done at 950 °C for 3 hours, the substrate was analyzed by SEM. From the SEM images, formations of growth layers with different morphologies, on the primary wetting film, were observed. NbB2 powder production was carried out on copper substrate, in a 1.5 m reactor at 950 °C. After nitric acid leaching, the efficiency of the production process was 97.3%. Since such a high efficiency was achieved in a single experiment, there was no need to experiment on different parameters. No matter which morphology its phases had, NbB2 was transformed into free particles after the copper base was leached away. From the copper nitrate solution obtained by the leaching operation, electrolytic copper powder was produced. This powder is an end product and it is more expensive than copper plates and copper folios. This way, the base material was not wasted away after leaching. In our laboratory, in the electrolysis cell produced from acrylic glass, reduction was carried out by using platinized titanium anodes and stainless steel cathodes. Since the beginning of the operation, cathodic current density was maintained at a level higher than the limit current density (>450 A/m2). At the end of the electrolysis operation, 99.9% pure electrolytic copper powder was produced. Boride powders, produced as free particles, were cold pressed (8 tons) in both pure form and together with sintering agents. After pressing, 4 mm thick green bodies were produced, which have a diameter of 12 mm. These green bodies were sintered in the Lynn furnace, at 1400-1750 °C, for 4-7 hours, without pressure. The dimensional changes and densities of sintered products were measured and their structures were visualized by SEM. Since these green bodies do not reach their theoretical densities, mechanical and electro-magnetic measurements could not be performed. It was decided to carry on with sintering experiments.