M.F. Zawrah

Effect of mullite formation on properties of refractory castables

In order to study the effect of the mullite bond phase on the physico-mechanical and refractory properties of the refractory castables, four castable samples were prepared; castable 1 contained 15 wt.% calcium aluminate cement (80% alumina) and 85 wt.% well graded tabular alumina . The other three samples (2, 3, 4) contained 10, 5 and 2% cement and 5, 10 and 13% of an alimina/ silica mixture (in the ratio 2.33:1, the molecular ratio of mullite). Their composition was investigated after firing at different firing temperatures using X-ray diffractin analysis. The physico-mechanical and refractory properties of such castables were determined according to standard specifications. Castable 4 that contained only 2 wt.% cement with 13 wt.% alumina/silica mixture and 85 wt.% well graded tabular alumina aggregate exhibited outstanding physico-mechanical and refractory properties after firing at 1500°C due to the presence of mullite in the bond phase with very little CaO. This enables their use in various refractory applications such as in steel, aluminium, copper, glass, cement, chemical and ceramic production.

Sinterability, microstructure and properties of glass/ceramic composites

Two series of glass/ceramic composites, glass/mullite (MG) and glass/spinel (SG) were prepared from borosilicate glasses and mullite or magnesium aluminate spinel. Both composites contain 50, 60, 70, 80 wt.% borosilicate glass. The formation of cristobalite in the glass matrix of low firing glass/ceramic composite substrates limits the efficiency of the ceramic substrate when it is used in circuit boards. In the present study, addition of mullite or spinel to a borosilicate glass as a ceramic filler caused the diffusion of mullite or spinel constituents into the glass matrix and prevented the formation of a cristobalite. Spinel suppresses cristobalite formation more effectively than mullite, and results in lower dielectric constant and thermal expansion coefficients.

Effect of surfactant types and their concentration on the structural characteristics of nanoclay.

A series of organo-modified nanoclays was synthesized using three different surfactants having different alkyl chain lengths and concentrations [0.5-5.0 cation exchange capacity (CEC)]. These surfactants were Ethanolamine (EA), Cetyltrimethylammoniumbromide (CTAB) and Tetraoctadecylammoniumbromide (TO). The obtained modified nanoclays were characterized by X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM) and compared with unmodified nanoclay. The results of XRD analysis indicated that the basal d-spacing has increased with increasing alkyl chain length and surfactant concentration. From the obtained microstructures of these organo-modified nanoclays, the mechanism of surfactant adsorption was proposed. At relatively low loading of surfactant, most of surfactant entered the spacing by an ion-exchange mechanism and is adsorbed onto the interlayer cation sites. When the concentration of the surfactant exceeds the CEC of clay, the surfactant molecules then adhere to the surface adsorbed surfactant. Some surfactants entered the interlayers, whereas the others were attached to the clay surface. When the concentration of surfactant increased further beyond 2.0 CEC, the surfactants might occupy the inter-particle space within the house-of-cards aggregate structure.

Synthesis and characterization of SiC and SiC/Si3N4 composite nano powders from waste material.

In the present work, nano silicon carbide has been prepared by pyrolysis of rice-husk ashes as starting materials. Three rice-husk ash samples having different features were used. The first was coarse-grained rice husk ash (fired husk as is), the second was fine rice husk ash (hand-ground), while the third was ball milled one. Effect of ball milling of the starting ashes for 6h on the formation of nano SiC was investigated and compared with those prepared without milling. The particle sizes of the prepared SiC materials were affected by the milling process. The particle sizes of the obtained nano SiC from ball milled staring materials were smaller than those prepared without milling. The pyrolysis conditions, i.e. the temperature and atmosphere were optimized. The optimum firing temperature to obtain well crystalline nano SiC was 1550°C. The effect of pyrolysis atmosphere, i.e. argon, vacuum and nitrogen was also demonstrated. The pyrolysis in argon exhibited lower efficiency on the formation of SiC than vacuum; while the pyrolysis in nitrogen atmosphere led to formation of SiC/Si(3)N(4) nanocomposite.

Microstructure and mechanical characteristics of laser-alloyed alumina ceramics

Commercially available alumina ceramic Al24 containing 5% porosity was alloyed by adding HfO2 or mixtures of HfO2 with 5 or 10 mol% SiO2, CeO2 or cordierite glass using infrared laser radiation. Thickness of the alloyed surfaces ranged between 500 and 800 μm depending on the composition of the alloyed powders and laser parameters used. The resulting microstructures and worn surfaces were analysed by electron microscopy. As a result of the alloying, the tendency of crack formation and grain growth was substantially reduced in the Al2O3 ceramic during laser treatment. The composite layers, showing no open porosity, contained 20–40 vol.% second phases, which were homogeneously distributed at the grain boundaries of the alumina matrix. Due to alloying, average grain size, hardness and Youngs modulus of the composite layers were reduced, but resistance to cracking under the mechanical action of a sliding diamond (scratch test) abrasive wearing were substantially enhanced compared with the unmodified dense alumina ceramics Al23.

Facile One-Pot Fabrication of Hollow Porous Silica Nanoparticles

A method for the fabrication of hollow silica nanospheres, a facile one-pot hydrothermal route, is described. Heating of an aqueous solution of water glass and D-glucose to 180 °C for 24 h affords-as indicated by transmission electron microscopy-a nanospherical composite consisting of a silica shell sheathing a carbonaceous core. Subsequent removal of the carbonaceous interior through oxidation in air produces hollow silica structures. Variation of the concentration of the two jointly dissolved chemicals enables a variation of the thickness of the silica shell. The hollow silica particles were characterized by means of SEM, TEM, XRD, IR spectroscopy, thermogravimetrical analysis (TGA), and sorption measurements.

Bauxite based low and ultralow cement castables

Abstract Five batches consisting of 90 wt-% graded bauxite aggregate with 10 wt-% of different cement matrices were investigated for the processing of low (LCC) and ultralow (ULCC) cement castables. The influence of variation in high alumina cement (HAC) content and fine alumina/fumed silica ratio on the physical and thermomechanical properties of the hydrated castables before and after firing at 1400°C was studied. The results were interpreted in the light of solid phase composition and microstructure, on the basis of XRD and SEM characterisation. As high alumina cement content decreases from 5·0 to 3·3 wt-% and fine alumina/fumed silica ratio increases from 1·5-2·3 to 2·3-9·1, in the matrices of LCC and ULCC respectively, a gradual decrease in green bulk density and cold crushing strength occurs. The inverse of this behaviour occurs after firing the hydrated samples for 2 h at 1400°C. All processed LCC and ULCC samples show high thermal shock resistance on successive quenching from 1000°C to ambient temperature, high volume stability on refiring for 2 h at 1500°C, and high loadbearing capacity on firing under load up to 1600°C. The temperatures corresponding to maximum expansion T0 and the beginning of subsidence T0 0·5 gradually increase from 1400 to 1500°C and from 1560 to > 1600°C respectively on increasing the fine alumina/fumed silica ratio from 1·5-2·3 in LCC to 2·3-9·1 in ULCC samples, owing to the formation of acicular mullite.

Preparation and characterization of barium containing refractory materials

Abstract Two grades of barium-containing refractory grains were processed from low and high purity materials by sintering methods after firing up to 1700°C. The first one was prepared from highly pure Al 2 O 3 and BaSO 4 , while the second one was prepared from raw Egyptian barite and Chinese bauxite. The densification parameters in terms of bulk density and apparent porosity were determined using the water displacement method. XRD was used for qualitative determination of phase composition, while SEM attached with an EDS unit was applied to study microstructure and microanalysis of the formed phases. The results reveal that grains processed from pure materials show BA 6 as a major phase with lower densification and open pores as compared with those prepared from commercial materials which exhibit higher densification and direct-bonded celsian/celsian and celsian/corundum phases with minor closed pores. The presence of higher amount of fluxing oxides, especially SiO 2 and Fe 2 O 3 in the latter one has contributed in lowering its densification temperature and refractoriness as well as increasing its cold crushing strength.

Investigation on the properties of rubber composites containing modified clay

Purpose – The purpose of this paper is to evaluate the efficiency of organobentonite (OB) as reinforcing filler in acrylonitrile-butadiene rubber (NBR). The composites were prepared using different loadings of OB and studying in details their properties. A series of OB was modified using surfactant N-cetyl-N, N, N-trimethyl ammonium bromide (CTAB) with concentrations 0.5, 1 and 2 cation exchange capacity (CEC) of bentonite. Design/methodology/approach – The different bentonites were characterized using different analytical and spectro-photometric techniques, such as infra red, X-ray diffraction, thermogravimetric analysis and scanning electron microscopy, while rubber vulcanizate rheological, morphological, swelling and thermal properties were examined using different standard instrumental testing and methods. Findings – The study revealed that the modification of bentonite using CTAB showed significant enhancement on NBR properties, and the optimum filler loading was 12 phr for both 0.5CEC OB and 2CEC OB...

Utilisation of Egyptian industrial waste material in manufacture of refractory cement

Abstract Egyptian calcium hydroxide byproduct, produced as a waste material during preparation of acetylene gas, causes various ecological problems. The present work had the aim of utilising this waste material as a source of calcium oxide in the production of refractory calcium aluminate cement. Two types of calcium aluminate cement (pure and commercial grade) were prepared, by mixing calcium hydrox ide waste material with pure calcined alumina and calcined bauxite respectively. T he cements were prepared by sintering at 1500-1550 °C for 3 h. The mineralogical compositions of anhydrous and hydrated cements were investigated by X RD. Also, thermal analysis and microstructure were used to identify the hydrated phases. The study revealed good cementing, sintering, mechanical, and refractory properties of the manufactured cements, enabling their use in high temperature applications (> 1500 °C) in the case of pure cement, and in moderate temperature applications (< 1500 °C) in the case of commercial cement.