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Showing 4 results for Mashhadi
Investigation of Molybdenum Disilicide, Boron Carbide and Silicon Carbide Particles on Flexural Strength and Pressureless Sintering Behavior of ZrB2-HfB2 Ultra High Temperature Composite
M. Mashhadi, A. Abdollahi, Z. Nasiri,
Volume 33, Issue 2 (Journal of Advanced Materials-fall 2015)
Abstract
In this study, ZrB2-HfB2 composite was produced by pressureless sintering method. MoSi2 B4C and SiC particles were used as reinforcement. ZrB2 powder was milled in planetary ball mill apparatus and then reinforcement particles were added to the milled powder. The composite powders were then CIPed and sintered at 2100oC and 2150oC. Scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS), flexural test, and resonance frequency method were used to compare the added particle effects on mechanical properties and pressureless sintering behavior of ZrB2-HfB2 composite. The analysis showed that the ZrB2-HfB2-MoSi2-SiCnano composite displays the largest gain in flexural strength. Furthermore, increasing the sintering temperature leads to an increase in flexural strength of samples.
The Effects of NH4Cl and Particle Size of Al Powder in AlN Whiskers Synthesis by Direct Nitridation
F. Meraji, M. Mashhadi, M. Tamizifar, A. Nemati,
Volume 34, Issue 1 (Journal of Advanced Materials-Spring 2015)
Abstract

In this study, AlN whiskers were prepared in a tube furnace at 1000˚C for 1h with 500 nitrogen gas flow. Al powders with particle size of 3 μm and 45 μm and NH4Cl were used as raw materials. SEM, TEM and XRD analysis were used to characterize AlN whiskers. The results showed that the diameters of AlN whiskers would range from 140 nm to 340 nm if different amounts of NH4Cl and 3 μm Al powder were used. In the case of using NH4Cl more than 40wt%, pure AlN without any unreacted Al was formed as the final product. Using NH4Cl and Al with particle size of 45 μm led to AlN whiskers with 630 nm to 870 nm in diameter. By adding 50%wt NH4Cl, pure AlN was formed. The diameter of the whiskers was increased by increasing NH4Cl content in starting materials (about 200 nm). Also, an increase in the diameter of AlN whisker resulted from coarse Al powder. By adding NH4Cl to Al, thermodynamically spontaneous cholororination - nitridation reactions were increased in vapor phase and whiskers and pure AlN powder were produced.


Synthesis of HfB2 Powders by Carbothermal Reduction
M. Shirvani, M. Mashhadi, M. Yosofi,
Volume 38, Issue 4 (Journal of Advanced Materials-Winter 2020)
Abstract
In this research, solid state carbothermal synthesis of HfB2 Nano powders was investigated. For this purpose, HfO2,H3BO3, carbon active or phenolic resin were used as raw materials for carbothermal reaction. After 2-4 hours of milling the raw materials by a planetary ball in an ethanol media, the mixture was heat dried and pressed in to disks under pressure of 20-30 bar. The disks were then placed in a graphite crucible and heat-treated at 1500–1600 °C for 1 hour under flowing Ar atmosphere. The powder product was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and backscattered electron (BSE) imaging. The heat treatment temperature, amount of boric acid, milling and heat treatment time were recognized as four factors influencing synthesis and size of HfB2 powder particles. SEM images reported irregular morphologies with a particle size of about 2-4 µm for HfB2 powder. The best sample was obtained at a molar ratio of HfO2 : phenolic resin carbon: boric acid = 1:5:5 at 1600 °C after 1 hour of heat treatment. XRD results confirmed synthesis of pure HfB2 with a crystallite size of about 60 nm.

Pressureless Sintering & Mechanical & Thermal Properties of ZrB2-ZrC-SiC Nanocomposite
I. Forooghi , M. Mashhadi,
Volume 39, Issue 4 (Journal of Advanced Materials-Winter 2021)
Abstract
Due to their unique features, Ultra-High Temperature Ceramics (UHTCs) have potential applications in aerospace, military and industry. ZrB2-SiC composite as one of these ceramics has been at the center of researches due to its attractive mechanical, thermal and oxidation resistance. In this study, the effect of ZrC addition on pressureless sintering behavior, mechanical, microstructural and thermal properties of ZrB2-SiC nanocomposite were investigated. For this purpose, micron-sized ZrB2 and ZrC powders and nano-sized SiC powder were used. ZrB2-20vol% SiC nanocomposites with addition of (3, 6, 9, 12, 15) vol% ZrC were sintered by pressureless sintering method at 2100 ºC. The results showed that the addition of ZrC improved relative density, hardness and fracture toughness of ZrB2-20vol% SiC nanocomposite. Optimum properties were obtained in a sample containing 12 vol% ZrC and the relative density, hardness and fracture toughness of this sample were reported to be 99.01%, 16.95 Gpa and 5.43 Mpa.m0.5, respectively. Thermal analysis of the samples showed that by adding ZrC, thermal diffusivity of this nanocomposite reduced. The highest thermal diffusivity at room temperature equaled 35.3 mm2 /s and was obtained for ZS composite.

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