PREPARATION OF ZrC BY SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS

 Zirconium carbide (ZrC) is one of the important high-temperature structure materials with various physical-chemical properties, such as high melting point (3 550℃), high hardness (25 GPa),high corrosion and wear resistance, and high electrical conductivity, which make it a promising candidate for many applications such as cutting tools, wear resistant components, crucibles in the mechanical industry, electrical field emitter tips and arrays,especially nuclear reactor core material due to its low neutron cross-section and good irradiation-resistance.Several methods for the preparation of ZrC powders have been reported, such as carbothermal reduction of ZrO2 at elevated temperature, sol–gel process,low-temperature synthesis by cyclic reaction of Mg in ZrO2–Mg–CH4,mechanical alloying and self-propagating magnesium reduction synthesis.Zirconium powder (purity 99%, 44 μm), activated carbon (purity 98%, 1 μm, Abbrev. AC), carbon black (purity 99.9%, nanoscale, Abbrev. CB), graphite (purity 99.999%, 2–15 μm, Abbrev. G) were used as reactants for SHS synthesis. The starting powders with a mole ratio of Zr to C of 1:1 were mixed in a planetary ball-miller for 3.5 or 5 h, using ZrO2 balls with diameter of 5 mm as mixing medium. NaCl as a diluent was added to zirconium and carbon black mixtures in 5%–30% (mass ratio). The mixture was dried at 60 ℃ for 24 h and compacted to form a cylindrical sample under a pressure range from 6 to 12 MPa. The sample was ignited by tungsten filament placed close to the top of the sample and was pyrolyzed in the furnace under continuous flow of argon gas. The combustion temperature was measured by W5–Re26 thermocouples, which were pre-located in a small hole in close vicinity to the center of the compact.[12] To remove NaCl, the synthesized powders are leached by distilled water.

The phase analysis of the product was carried out by a X-ray diffractometer (XRD, Model Ultima , Ⅲ Rigaku, Japan). Morphology of the powders was examined by a scanning electron microscope (SEM, Model S–3400N, Hitachi). Crystal vibration and molecular structure were analyzed by a Raman spectroscope (Model inVia). An oxygen analyzer (Model Leco TC600), a carbon–sulfur analyzer and an inductively coupled plasma atomic emission spectrometry (ICP–AES, Model 4 300 DV, Optima) were used to analysis the chemical composition of the product. The particle size distribution of sample was determined by an image analysis system (Model DS–5M, Japan).XRD patterns shown in Fig.2 of the synthesized powders with different milling time of raw powders show only crystalline ZrC peaks. The ZrC diffraction peaks are narrow and sharp, indicating that there are large crystallites and order in the lattice.

 

Conclusions 

(1) ZrC powder was prepared by self-propagating high-temperature synthesis based on the reaction between zirconium and carbon. The highest combustion temperature of zirconium and activated carbon system is 2 446 K with compacting pressure of 8 MPa, milling time of 5 h. 

(2) Carbon black is believed to be the best carbon source for preparing ZrC powder. The mean particle size of the product synthesized from zirconium and carbon black is 0.5–1 μm and the oxygen content is 0.38%. 

(3) The combustion temperature between reaction of the zirconium and carbon black with 30% NaCl decreases to 1 810 K. With the increase of NaCl, the particle size of powder decreases, and its mean particle size is about 50nm when NaCl content is 30%.

 

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