جستجوی مقالات مرتبط با کلیدواژه "کم سیمان" در نشریات گروه "مواد و متالورژی"

Cr2O3, YSZ and SiC nano-powders were prepared by ball milling and subsequently mixed and agglomerated to reach the proper composition and size for spraying. Morphological investigations and particle size analysis showed that powder particles after 5h of milling time reached to an ultrafine/nano size. The powder mixtures were then deposited onto 304L steel substrates using Atmospheric Plasma Spray (APS) to deposit Cr2O3-20vol%YSZ-10vol%SiC composite coating. Microstructure and morphology of the elemental/milled powders and plasma sprayed coatings were characterized using Field Emission scanning electron microscopy (FESEM) equipped by EDS. X-ray diffraction (XRD) patterns of powder particles and coatings included only the elemental peaks without any traces of impurities and new appearance peaks. Using image analysis method, the coatings porosity content increased from 8.7 to 12.8 through formation of composite coating. Also mechanical properties of the coatings including bonding strength, micro hardness and fracture toughness were evaluated. The results showed that coatings have high bonding strength of 40-49 MPa. Regarding hardness and toughness results, adding reinforcements to Cr2O3 coating increased hardness and toughness to 910 HV and 8.1 MPam1/2, respectively.

In this study, the effect of nano-alumina addition on the properties of high-alumina low-cement self-flowing refractory castables was studied. For this reason, the reactive alumina in the refractory castable composition was substituted by nano-alumina powder. Then, the self-flow characteristics such as; self-flow value and working time of high-alumina low-cement refractory castables were studied. Besides, the physical and mechanical properties, phase composition and microstructure of these refractory castables were studied after drying at 110 °C and firing at 1250 and 1450 °C. The results showed that the nano-alumina addition has a great effect on the self-flow characteristics and mechanical strength of these refractory castables. With addition of nano-alumina particles, the self-flow value and working time tends to decrease due to high surface area of nano-alumina particles. By use of 1 wt.% nano-alumina content in the castable composition, the alumina self-flowing castable with adequate working time can be obtained according to standard values of self-flowing castables. The decrease in porosity and, increase in mechanical strengths after drying is obtained by adding nano-alumina. The phase analysis and microstructure evaluations showed that CA6 phase can be form at lower temperatures (1250 °C) with the addition of nano-alumina. CA6 platy formation leads to increase of porosity in the microstructure. But, the because of bonding behavior of platy CA6, the mechanical strengths are considerably increased after firing.

A basic alumina-spinel low cement castable (castable A) and with 5% addition of sodium hexa meta phosphate (castable B) are prepared and heat treated at 110ºC، 900ºC and 1400ºC. It was shown that after heat treating at 110ºC، Cold Crushing Strength (CCS) of castable B is more than 3 times and Apparent Porosity (AP) less than half of the castable A. Presence of Mg2P2O7 and AlPO4. 2H2O is the main difference. At the temperature range of 800-1000ºC that hydraulic bond reverses to dehydrate condition and castable A becomes weak with high porosity، castable B shows a CCS of more than 4 times. Needles of magnesium phosphate are responsible for reinforcing microstructure of castable B at 900ºC. After firing at 1400ºC، castable B showed extra ordinary values of CCS more than 100 MPa (about 1000 kg/cm2). Reasons are discussed by X-ray diffraction and scanning electron microscopy studies.

In this paper, the effect of nano-magnesia particles on the self-flow characteristics of high-alumina low-cement self-flowing refractory castables has investigated. For this reason, reactive alumina is substituted by nano-magnesia particles in 0-2 wt.% range and the flow and setting behavior of these castables have tested. Also, major physical (B.D and A.P), mechanical properties (C.C.S and M.O.R), phase analysis (XRD) and microstructure (SEM) of these castables have evaluated. The results showed that with increase of nano-magnesia particles with high surface area, the self-flow value and working time tend to decrease. Also, the mechanical strength of these castables after drying is enhanced with increase of these particles in castable composition by pores filling between coarse particles. As a result of using nanometer-sized magnesia particles with high surface area, the higher amount of spinel phase can be formed at lower temperatures 1250 °C). Increasing of porosity and then, decreasing of strength can occur with formation of expanding spinel phase.