LEE- Malzeme Bilimi ve Mühendisliği-Doktora
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Yazar "Özkan Aytekin, Nihan" ile LEE- Malzeme Bilimi ve Mühendisliği-Doktora'a göz atma
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ÖgeSynthesis and characterization of various tungsten carbide powders from tungsten hexachloride powders via mechanochemical reaction and autoclave/pressure vessel methods( 2020) Özkan Aytekin, Nihan ; Öveçoğlu, Mustafa Lütfi ; 632586 ; Malzeme Bilimi ve MühendisliğiTransition metal carbides have been studied for approximately last half century due to their extraordinary characteristics that make them convenient to be utilized for industrial applications such as tool and structural materials under special conditions. Tungsten carbide has a special location amongst transition metal carbides because of its superior properties such as high hardness, high density, high melting temperature, high fracture toughness, good electrical and thermal conductivity and high elastic modulus etc. These attractive properties make it preferred to be used for tips for cutting and drilling tools, WC-Co hard metals, wear-resistant surfaces for machines, scratch-resistant jewelry materials, erosion-resistant coatings for aerospace elements, extrusion and pressing molds, wear resistant parts in wire drawing, platinum-like catalytic materials for polymer electrode membrane fuel cells and thin film diffusion barriers in microelectronics. In W-C binary system, two phases such as W2C and WC exist. These two phases possess various polymorphic modifications that are stable in different temperature and composition ranges. Many production techniques have been used in the sythesis of tungsten carbides. The production of WC was firstly achieved in a conventional way by high temperature. A solid state direct reaction occured between W and C powders at a temperature range of 1200°C-2000°C under controlled atmosphere to synthesize WC. Also there are different high temperature methods such as carbothermal reduction, chemical vapour condensation, self propagating high-temperature synthesis, molten salt synthesis, calcining and thermal processing at high temperatures. In addition to these techniques there are relatively low temperature methods such as low temperature autloclave processing, combined process of calcination, nitridization and and carburization at low temperatures, electrochemical sytnhesis from halide-oxide melts under pressure, mechanically alloying and mechanochemical synthesis. Especially mechanically alloying and mechanochemical synthesis are advantegous methods due to having simple and low-cost equipment and performing reactions at room temperatures in relatively short reaction times. The main aim of this dissertation is to produce WC powders with an effective technique that overcomes the limitations of traditional production methods and provides advantages of time, energy saving, simplicity and low equipment cost. In order to achieve this aim, elemental W and C powders were first milled in a spex at different durations by mechanical alloying method to obtain tungsten carbide powders. The aim of the first stage is to obtain the tungsten carbide powders with the starting powders traditionally used, and to compare the results with the results of the study in which tungsten carbide was synthesized with the alternative starting powders. In the second stage, mechanical chemical reaction was used to synthesize high purity and sub-micron sized WC powders using WCl6 (as W source), Na2CO3 (as C source) and Mg (as a reducing agent) powders. In a very short time the chemical reaction started and the WC phase occurred. In a very short time chemical reaction started to form and WC phase occured. Amounts of C source (Na2CO3) and reducing agent (Mg) were varied to optimize raw materials reacted and to obtain high-purity WC powders. After purification of the obtained powders, nano-sized tungsten carbide powders, which are the only WC phase, were achieved in high purity without any intermediate phase and impurity. The other purpose of this dissertation is to use an alternative production technique in order to produce WC powders, and compare the products, especially those produced by mechanical alloying / mechanochemical synthesis and autoclave methods. Therefore in the third stage low temperature autoclave processing was employed for the fabrication of WC powders from the same raw materials. The autloclave processing was used with mechanical activation to reduce the reaction temperature by activating the reactive particles and to homogenize the distribution of the microstructure. In this method powder blends were placed in a hydrothermal reactor and the hydrotermal reactor was heated in a furnace. This autoclave processing was implemented by varying the temperature and the duration of synthesis, the type of excess carbon source, the amounts of carbon sources and reducing agent. High purity and nano-sized tungsten carbides were obtained by optimizing the production conditions. Furthermore, autoclave synthesis is accepted as an eco-friendly method due to carrying out in a closed / isolated system and saving energy. The final powders of two different methods were characterized and compared each other using X-ray diffractometer (XRD), particle size analyzer (PSA), pycnometer, stereo microscope (SM), scanning electron microscope/energy dsipersive spectometer (SEM/EDS), transmission electron microscope (TEM) and differential scanning calorimetry/thermogravimetric analysis(DSC/TGA). WC powders were produced from WCl6-Na2CO3-Mg powder blends via mechanochemical synhtesis for the fist time.in the literature. In this way the results of mechanochemical snthesis from new raw materials for WC powder production contibuted the literature. Also, this thesis provides a comparison of mechanochemical method which is a simple, energy saving and room temperature method and autoclave method which is a more traditional method for producing tungsten carbide.