Key Engineering Materials Vol. 608

Abstract: Cordierite (2MgO·2Al₂O₃·5SiO₂) has a low density of 2.2 g/cm³ due to its high magnesium content. Because of its low thermal expansion coefficient of 1~2 x10^-6 /^°C, many studies are being conducted on the synthesis of cordierite with the expensive petalite as a replacement for lithium alumina silicate-based heat-resistant materials. The cordierite can be synthesized over a wide range: SiO₂ at 50~70%, Al₂O₃ at 20~40%, and MgO at 10~30%. In this study, the range of chemical composition and temperature of cordierite synthesis is thoroughly investigated. In particular, we use natural materials (magnesite, kaolin and clay) to examine how thermal properties are affected by changes in crystal phase arising from the varying composition of MgO, SiO₂ and Al₂O_3. We focused on factors leading to an increase in the rate of cordierite synthesis at temperatures below 1280 ^°C. From observing the synthetic rate over 1250~1280 ^°C, the sintered body at 1280 ^°C had a high synthetic rate greater than 80%. Magnesite was ball milled at constant intervals, and mean particle size was controlled to improve the synthetic rate of cordierite. As a result, the cordierite synthetic rate increased by more than 15% with decreasing the mean particle size of magnesite.

Abstract: ZnO nanowalls were synthesized by chemical vapor deposition at temperature of 650 °C for 1 hour on the silicon substrate. The morphologies of samples were characterized by scanning electron microscopy (SEM). The result from X-ray diffraction (XRD) confirmed that the ZnO nanowalls were vertical c-axis orientation. A room temperature Photoluminescence peak at 378 nm is ultraviolet emission (UV) and the broad peak at wavelengths around 450-650 nm is corresponding to the green emission of ZnO nanostructure. This synthesis may be applicable for gas sensor or solar cells.

Abstract: Porous concretes such as aerated and lightweight concretes are commonly used in construction fields. Lightweight construction materials are used to reduce either the weight or the budget of building structures. Porous concrete production is widely utilised aluminium (Al) powder to increase pores in concrete structures and giving information for porous geopolymer production. It was introduced by adding 0.05-1% Al-powder as the initiated materials of geopolymers, to react with water in those materials and promote hydrogen gas inside specimens. The research, therefore, focused on the synthesis of porous geopolymer by metakaolin as a pozzolan and mixed with alkali solution (8M NaOH and Na₂SiO₃) as well as Al-powder as a foaming agent. The highly porous geopolymers were produced with various Al-powders as 0%, 0.2%, 0.4%, 0.6% 0.8% and 1% by weight. After 7, 14 and 28 days age, the specimens were tested the mechanical properties, such as compressive and flexural strengths. The water absorption, apparent porosity and bulk density were analyzed at 28 days age. The synthesis of metakaolin-based porous geopolymers with Al-powder presented good results. It showed that Al-powder content affected to degree of porosity of geopolymers. Keywords: Metakaolin based geopolymer, Porous geopolymer, Aluminium powder, Foaming agent, Mechanical and physical properties

Abstract: The objective of this study is to explore a method to improve hydrophilicity of the surface of formers that could be more easily dipped to produce high quality dipped rubber products. Photocatalytic TiO₂ composite films were prepared by sol-gel method. Glass formers were coated with nitrogen doped TiO₂ nanocomposite film by dipping method and annealed at 600°C for 1 h. Phase formation of TiO₂ was characterized by XRD. Morphology of the TiO₂ films was observed by using atomic force microscopy (AFM). Optical absorption of the films and degradation concentration of methylene blue had been measured employing UV-Vis spectrophotometer. The hydrophilicity of the thin films was determined in terms of the contact angles of water and NR rubber latex droplets on the coated TiO₂ composite films under visible light irradiation. It was found from the experimental study that the TiO₂ nano-composite film can improve wet-ability of the coated former surfaces.

Abstract: Cr and La-codoped SrTiO₃ nanoparticles were successfully prepared by a microwave-assisted hydrothermal synthesis. The products were characterized by XRD, TEM, BET specific surface area, and UV-Vis absorption spectra. The photocatalytic activity was determined by deNO_x ability under irradiation of a 450 W high-pressure mercury lamp. The products could absorb the visible light and showed high photocatalytic ability for the decomposition of NO_x gas. All samples showed the photocatalytic activities superior to that of commercial titania (Degussa P25) in both UV and visible light regions.

Abstract: This study investigated the effect of aluminium dopant on ZnO morphologies synthesized by microwave assisted method. The precursors used were Zn (NO₃)₂.6H₂O, NaOH and AlCl₃.6H₂O. The conditions for microwave heating were 160 and 640W, 10 min with 5s ON/15s OFF step. Microwave assisted heating generates heat to activate the formation of ZnO. Phase study was carried by X-ray diffraction which confirmed the formation of wurtzite structure of ZnO for all synthesized conditions. Microstructure was studied by a scanning electron microscope which revealed different crystal morphologies of ZnO at different amount of Al doping concentrations. ZnO rod was about 3-5 μm in length and 200 500 nm in cross section with hexagonal crystal plane. During microwave heating, nucleation of ZnO rods started at a point leading growth into larger flowers. In this work, it was suggested that Al acted as a nucleating agent and affected the c-axis orientation for ZnO crystal growth. Small needles/rod-like crystals were observed probably due to fast nucleation and a decrease of growth on (002) plane.

Abstract: ZnO is a good candidate material for many optical and optoelectronic applications. ZnO with various shapes and sizes can be prepared via chemical methods such as precipitation, microwave heating and hydrothermal method. Generally ZnO synthesized by hydrothermal method uses autoclave which is expensive and gives low % yield. This research applied a low cost high pressure cooker which replaced the use autoclave to synthesize ZnO as its concept is similar to hydrothermal method. In this study, it was found that the size and shape of the synthesized ZnO particles were affected by several factors such as Zn²⁺/OH^- ratio, temperature and time. Zinc nitrate hexahydrate, Zn(NO₃)₂.6H₂O and sodium hydroxide, NaOH were used as metal ion sources in the precursor solutions. Structural and morphological studies were performed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The effect of Zn²⁺/OH^- ion ratios, hydrothermal temperature and time on the size and morphology of ZnO were discussed in detail. All the synthesized conditions yielded hexagonal wurtzite structure of ZnO confirmed by XRD, without calcinations process. SEM images showed plate-like structure for Zn²⁺/OH^- ratio = 1:7.5 and 1:15 and flower structure for Zn²⁺/OH^- ratio = 1:20. Sizes of the synthesized ZnO particles decreased with increasing hydrothermal temperature from 120 to 200°C. The longer the synthesized time period the larger the ZnO particles obtained.

Abstract: The Fe³⁺ and N–doped 3SnO₂/TiO₂ composite thin films and undoped films coated on glass fibers were prepared by sol–gel and dip–coating methods. The films were calcined at 600°C for 2 hour and characterized by XRD, SEM, EDS and XPS. The photocatalytic activity of the coated glass fibers was determined by means of degradation of a methylene blue (MB) solution and humic acid (HA). It was found that the optimized 20N/3SnO₂/TiO₂ composite films exhibit a high photocatalytic activity and HA could be rapidly removed from water. The main factor affecting the HA degradation of 20N/3SnO₂/TiO₂ films is quantity of glass fibers loading, irradiation power of UV lamp and flow rate of water.

Abstract: This work investigated the influence of specimen dimensions and temperature on the debinding behavior of alumina feedstock for powder injection process. Polyethylene glycol (PEG) was used as a based binder. It is soluble in water therefore the use of wax-based binder system can be avoided. PEGs with a molecular weight of 1500 and 4000 were mixed with polyvinyl butyral (PVB) a minor component. Stearic acid was added during feedstock preparation to act as a lubricant. Debinding process was carried out using a water leaching test at 30 and 70 °C to study for PEG removal rate. Specimens retained their shapes after leaching of the binders. In general, the rate of binder removal increased at initial stage then slowed down at longer leaching times. A faster debinding rate was achieved at 70 °C when compared to 30 °C. In addition, samples with a higher surface area to volume ratio (As/V) led to an increase in the %PEG removal rate. This study also showed the relationship between %PEG removal as a function of different surface area to volume ratio (As/V) which can be helpful in predicting %PEG removal for any other samples having cylindrical shape.

Abstract: The relationship between granule property, pressed green density and shrinkage of 92% alumina spray dried granules were studied for both simple and complex shapes. Two types of granules, the in-house granule (A) and the commercial granule (B), was observed morphology using microscopy and liquid immersion techniques. Examination of the granules showed that granule A have several undesirable features; agglomeration, hollow granules and non-spherical granules where granule B showed them to be unagglomerated and spherical. The mixture A and B were conducted to study the particle size distribution (PSD) and compared with Dinger-Funks ideal PSD. The result showed that granule mixture A: B 100:0 had closest PSD curve fitted but had lowest tapping and pressed green density than 70:30 and 30:70 mixtures. This is because the agglomerated shape in granule mixture 100:0 caused air gap in between granule contact resulting in low tap density. The specific fracture strengths of the granules can affect to the densification of green ceramic during pressing. Low pressed green density affected to a high amount of shrinkage during sintering and leading to small grain growth after sintered. The simple and complex shape follows the same trend in shrinkage.